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U.S. Department

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DOT/FRA/ORD-09/04 April 2009 This document is available to the

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North American Joint 

Positive Train Control Project

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Alan Polivka, Bill Moore Ede, and Joe Drapa

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Transportation Technology Center, Inc. Railroad Research Foundation 

P. O. Box 11130 50 F Street NW, Suite 5200 

Pueblo, CO 81001 Washington, DC 20001

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U.S. Department of Transportation DOT/FRA/ORD-09/04 

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This document is available through National Technical Information Service, 

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13. ABSTRACT

PTC offers the promise of significant potential benefits in railroad safety, capacity, and efficiency. However, PTC reveals new 

and much more complex design issues than those encountered with conventional train control systems. This is largely because a 

PTC system comprises a large, distributed, real-time communications, control, and mobile computing network that embodies 

and enforces many of the railroad operating rules. 

This report summarizes key issues encountered in developing the vital NAJPTC system along with solutions, rationale, and 

results. The unique experiences gained from this project have benefited other PTC projects and have led to the inception of 

subsequent projects to further address issues identified. 

15. NUMBER OF 

PAGES 

77 

14. SUBJECT TERMS Train control, communications-based train control, CBTC, positive train control, PTC, railroad 

safety, moving block

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ABSTRACT 

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i 
ii 
Table of Contents 

List of Figures.................................................................................................................... iv 

Acknowledgements............................................................................................................. v 

Executive Summary............................................................................................................ 1

1.0 Background.................................................................................................................. 5 

1.1 Project Objectives................................................................................................. 5 

1.2 PTC System Overview ......................................................................................... 5 

2.0 Definitions.................................................................................................................... 9 

3.0 Benefits from the NAJPTC Project............................................................................ 14 

3.1 Potential Headway and Capacity Benefits of Vital, Moving Block PTC........... 14 

3.2 Additional Benefits Provided by Vital, Moving Block PTC.............................. 18 

3.3 Benefits Derived from the NAJPTC Project ...................................................... 18 

4.0 Design Challenges ..................................................................................................... 20 

4.1 Location Determination, Track Discrimination, and Entry Point Protection ..... 20 

4.2 Consist Determination and Verification ............................................................. 23 

4.3 Mobile Radio Communications.......................................................................... 30 

5.0 Lessons Learned......................................................................................................... 36 

6.0 Conclusions................................................................................................................ 45 

Appendix A. NAJPTC Onboard Equipment.................................................................... 47 

Appendix B. Status of NAJPTC Implementation by Function........................................ 55 

Appendix C. Quantification of Potential Increased Capacity and Reduced Headway 

 with Vital, Moving Block PTC.................................................................. 59 

Appendix D. Deliverables................................................................................................ 73 

Acronyms ....................................................................................................................... 76 

iii 
List of Figures 

Figure 1. Testing of NAJPTC was in the State of Illinois................................................. 6 

Figure 2. PTC System Architecture ................................................................................... 7 

Figure 3. Moving Block has the Potential to Reduce Headways....................................... 8

Figure 4. Relationship of Moving Block and Conventional Signaling Parameters used in 

Analysis of Potential Headway Reduction and Capacity Increase for a Hybrid 

Moving Block/Fixed Block PTC System ...................................................... 16

Figure 5. Potential Headway Reduction and Capacity Increase for a Hybrid Moving 

Block/Fixed Block PTC system vs. 4-Aspect Signaling, for a Typical Grain 

Train............................................................................................................... 17 

Figure 6. Potential Headway Reduction and Capacity Increase for a Vital, Moving Block 

PTC System vs. a 4-Aspect Signaling System for a Typical Grain Train ....... 17 

Figure 7. NAJPTC Onboard Location Determination System ........................................ 22 

Figure 8. Simulated Braking Distribution Shows that an Average Loaded Grain Train 

Running at 60 mph Would Be Stopped 1,762 ft before Necessary by the PTC 

Braking Algorithm with “Confirmed” Consist ................................................ 28 

Figure 9. With Worst-Case Consist Assumptions, No Overruns Occur, But a 10–30 mph 

Train Stops within 500 ft of the Target only 2.7% of the Time....................... 29 

Figure 10. With Worst-Case Consist Assumptions, No Overruns Occur, But a 30–60 

mph Train Never Stops within 1,000 ft of the Target.................................... 29 

Figure 11. With Confirmed-Consist Assumptions, No Overruns Occur, But a 10–30-mph 

Train Stops within 500 ft of the Target only 79% of the Time ..................... 30 

Figure 12. With Confirmed-Consist Assumptions, No Overruns Occur, But a 30–60 mph 

Train Stops within 1,000 ft of the Target only 38% of the Time .................. 30 

Figure 13. Key Information Flows in PTC...................................................................... 31 

Figure 14. Example of TPM for POS Failures and Nonexplicit Mode Events................ 43 

Figure 15. Example TPM for False Reports of Authority Violation ............................... 44 

iv 
Acknowledgements 

Transportation Technology Center, Inc. (TTCI), a wholly owned subsidiary of the 

Association of American Railroads (AAR), managed the North American Joint Positive 

Train Control (NAJPTC) project under contract to the Railroad Research Foundation 

(RRF). RRF received funding for the project from the Federal Railroad Administration 

(FRA) under Cooperative Agreement No. DTFR53-03-H-00015. Additional funding was 

received from AAR and IDOT. The system engineering effort was led by ARINC and 

included team members CANAC, Battelle, and Parson Transportation Group. The PTC 

System Developer/Integrator team was led by Lockheed Martin, and included Wabtec, 

Union Switch and Signal, Parsons-Brinckerhoff, and the University of Virginia. The 

NAJPTC management committee included representatives from the Union Pacific 

Railroad (UP), CSX Transportation, Norfolk Southern (NS), BNSF Railway, Canadian 

National Railway, Canadian Pacific Railway, and Amtrak, and the AAR, FRA, IDOT, 

Volpe Center, ARINC, Battelle, Lockheed Martin, Wabtec, and TTCI. Stakeholders 

included representatives from the UP, NS, Amtrak, the AAR, FRA, IDOT, Lockheed 

Martin, and TTCI. 

v 
Executive Summary 

The North American Joint Positive Train Control (NAJPTC) project (a.k.a. “Illinois 

Department of Transportation Positive Train Control (IDOT PTC)” project) was an 

ambitious multiyear project to develop, test, and demonstrate a vital, interoperable, and 

cost-effective communication-based train control (CBTC) system that could improve 

safety and provide operational benefits as compared with conventional systems. The 

Federal Railroad Administration (FRA), Association of American Railroads (AAR), and 

IDOT jointly funded the project, and Transportation Technology Center, Inc. (TTCI), a 

wholly owned subsidiary of the AAR, managed the project. 

The NAJPTC system was developed and tested on a 120-mile corridor of the Union 

Pacific (UP) railroad in Illinois that hosts both freight trains and Amtrak passenger trains. 

In order to demonstrate overlay and standalone (moving block) methods of operation, the 

120 miles of PTC territory was divided into two categories of roughly 60 miles each: 

“PTC Integrated Territory” and “PTC Standalone Territory.” PTC Standalone Territory 

was to demonstrate the capacity improvements achievable with moving block operation. 

Trains on PTC Integrated Territory were constrained by conventional fixed signals, so it 

would not show capacity improvements but was intended to demonstrate efficient 

handling of nonequipped trains mixed with PTC-equipped trains. 

The NAJPTC project addressed the design challenges faced by PTC systems in general 

and provided valuable lessons learned. Through its design, development, and testing, the 

NAJPTC system has also aided other PTC projects in a number of ways as described in 

this report. Subsequent projects have been started that leverage off the NAJPTC project 

or that address issues encountered on the NAJPTC project requiring further development. 

This report presents the objectives, approach, results, benefits, and lessons learned from 

the NAJPTC project. 

Goals and Objectives 

PTC is a form of communication-based train control (CBTC). To satisfy the three basic 

safety characteristics specified by the FRA’s Railroad Safety Advisory Committee’s PTC 

Working Group, a PTC system must: 

o Prevent train-to-train collisions, 

o Enforce speed restrictions and temporary slow orders, and 

o Provide protection for workers and their equipment operating under specific 

authorities. 

PTC functions are modular to allow tailored implementation for each railroad or territory. 

Functionality negotiation capabilities permit trains to automatically adjust to the specifics 

of each deployment infrastructure. This, along with an open architecture, facilitates 

interoperability (operating one railroad’s locomotive on another railroad’s track under 

1 
PTC) and migration (supporting different levels of functionality at various stages of 

evolutionary implementation). 

In addition to the safety objectives listed, the complex NAJPTC system would enable 

increasing passenger train speed to 110 mph. In Standalone Territory, it would also have 

the potential to increase railroad capacity (reduce excess headway as compared with fixed 

block signaling) by means of its moving-block architecture. In Integrated Territory, 

however, it would operate in conjunction with fixed wayside signals to better 

accommodate a mix of equipped and unequipped trains, but with negative impact upon 

capacity as compared with that of the conventional signaling system. 

The moving-block concept allows a train to receive a movement authority between any 

two locations, rather than being constrained to the fixed block boundaries of conventional 

signaling. Movement limits are updated automatically and more frequently than in 

conventional systems. This potentially allows train spacing (headway) to be reduced and 

track capacity to be increased in following move (fleeting) scenarios and certain 

“bottleneck” situations, such as where track has been downgraded or returned to service 

after maintenance. 

Standalone PTC also has the potential to decrease train control system life cycle cost by 

reducing the amount of wayside vital equipment required (signals and track circuits). 

The NAJPTC project also tackled many of the most challenging issues in the 

development, refinement, and testing of PTC, including: 

o How to determine train location and turnout decisions with safety-critical 

dependability, 

o How to predict enforcement braking distance accurately enough that a train will 

not go past the target, but not be stopped far short of the target either, 

o How to verify train consist characteristics, and 

o How to meet PTC system performance requirements given the performance 

limitations of mobile radio communications. 

Summary of Benefits 

Benefits derived from the NAJPTC project are: 

o By integrating inertial sensors with DGPS, tachometer readings, and map 

matching, NAJPTC’s onboard location determination system increased the ability 

to dead reckon in areas lacking satellite coverage (such as in tunnels or “urban 

canyons”), significantly improved track resolution ability, and improved the 

detection of failure modes over systems that do not incorporate inertial sensors. 

o Portions of the NAJPTC system’s hardware and software implementation have 

been reused by other PTC projects. 

2 
o Open specifications/design documents from the NAJPTC project have been 

utilized in other PTC projects. 

o The NAJPTC system made greater use of commercial, off-the-shelf hardware and 

software components than prior vital train control systems. 

o NAJPTC was the first project to apply new FRA rule (Title 49 Code of Federal 

Regulations Part 236, Subpart H). Its safety case (Product Safety Plan, or PSP) 

provided a template and source material for other PTC projects. 

o Being the first to attempt many new and complex train control approaches, 

NAJPTC testing in Illinois identified a number of design challenges for which 

solutions were developed and from which valuable lessons were learned 

(summarized below), benefiting other PTC projects. 

Design Challenges and Solutions 

Location Determination, and Track Discrimination and Entry Point Protection. The 

NAJPTC system uses a combination of technologies to address these issues. NAJPTC’s 

location determination system features a multiple-sensor, inertial navigation system (INS) 

design with diverse self-checking for safety. When temporarily out of DGPS signal 

coverage, the INS components allow the system to continue operating by dead reckoning. 

Consist Determination and Verification. The NAJPTC system design addresses the 

problem of uncertain freight train consist characteristics by using two modes: 

Unconfirmed and Confirmed. When a train is initialized, it enters Unconfirmed Consist 

mode. When sufficient data becomes available for NAJPTC to verify the consist 

characteristics with adequate integrity for safety-critical purposes, it transitions to 

Confirmed Consist mode. 

Mobile Radio Communications. Based on preliminary analysis and requirements, the 

Advanced Train Control System (ATCS) Specification 200 standard was the RF data link 

selected for the NAJPTC project. Evolving requirements proved the 1980’s-era ATCS 

data radio system’s performance insufficient for NAJPTC’s needs. Various potential 

solutions were assessed, and ultimately a separate project was initiated to specify the 

requirements, to develop, and to test a new generation interoperable higher performance 

data radio system suitable for use with PTC systems. 

Lessons Learned 

The NAJPTC project was one of the most complex system development/integration 

projects undertaken to date. The complex, highly distributed system included mobile 

nodes and was required to meet stringent safety, availability, and performance 

requirements. The requirements for NAJPTC far exceeded those of any train control 

system previously implemented. 

In addition to invaluable technical results described in this report, other lessons learned 

include: the necessity for incremental development of such complex systems; the need for 

3 
thorough and unambiguous specifications; ensuring that the system developer thoroughly 

understands the system requirements as well as the underlying rationale and train 

operations; open communication and a cooperative working relationship between the 

railway and system developer; early test planning; proper rigorous sequence of 

development steps; having a productive test environment; milestones to demonstrate 

visible and quantifiable progress during development; maintaining focus on system 

performance; the need for more adaptive and robust braking algorithms. 

Critical Milestones 

The NAJPTC project was a multiyear effort with phased development milestones staged 

according to software builds, each of which adds features to the previous one: 

o Build 1: Location determination, reporting, and tracking; Office, onboard, and 

communications infrastructure. 

o Build 2A: PTC Integrated Mode operation at 79mph; Track bulletins; Predictive 

and reactive enforcement. 

o Build 2B: PTC Standalone Mode operation; Crossing advance activation; Highspeed (110 mph) passenger train operation; Upgraded RF communications. 

o Build 3: Verification of consist length and weight; Pacing; Roadway worker 

terminal; Functional negotiation; Integral defect detectors; Emergency braking 

enforcement; Display of predictors; Online track database update capability; and 

Cab signal interoperability demo. 

Due to the difficult design challenges, the project was terminated before the completion 

of Build 2A because IDOT wanted to begin high-speed operation sooner than would be 

achievable with the PTC approach. Consequently, Builds 2B and 3 were not fully 

implemented and tested, although requirements and much of the design was done. 

Conclusions 

CBTC offers the promise of significant potential benefits in railroad safety, capacity, and 

efficiency. This report has identified key issues encountered on the NAJPTC project, 

along with chosen solutions and lessons learned. The experiences gained from this 

project have benefited other PTC developments and have led to the inception of 

subsequent projects to further address issues identified on the project. 

4 
1.0 Background 

The Federal Railroad Administration (FRA), Association of American Railroads (AAR), 

and the Illinois Department of Transportation (IDOT) jointly funded the North American 

Joint Positive Train Control (NAJPTC) project, and TTCI, a wholly owned subsidiary of 

the AAR, managed the project. The project is also known as the Illinois Department of 

Transportation Positive Train Control (IDOT PTC) project. 

1.1 Project Objectives 

The NAJPTC project had the following stated objectives: 

• Develop, test, and demonstrate PTC capabilities, including moving block 

operations, interoperability, and advance activation of highway crossing 

devices in a corridor with both freight and passenger train service. 

• Meet the safety objectives of: 

− Preventing train-to-train collisions, by preventing trains from violating 

their authority limits, 

− Enforcing speed restrictions, including civil restrictions and temporary slow 

orders, and 

− Protecting roadway workers and their equipment operating under specific 

authorities, by preventing trains from entering work limits before the train 

crew has acknowledged that they have contacted the employee in charge 

(known as EIC). 

• Provide for industry interoperability (between railroads or different territory 

types within a railroad), demonstrate safe operation of locomotives equipped 

with interoperable systems, and enable equipped trains operating from 

different railroads to enter a foreign railroad at track speed. 

• Provide a cost-effective design to enhance prospects for deployment. 

1.2 PTC System Overview

The NAJPTC system was installed and tested on a 120.7-mile segment of the Union 

Pacific (UP) railroad in Illinois, referred to as the NAJPTC line, as Figure 1 illustrates. 

This line is part of IDOT's proposed high speed rail corridor extending from Chicago on 

the north to St. Louis on the south, a distance of 280.5 miles. The segment selected for 

the NAJPTC system extends from south of Mazonia, Illinois, (milepost 62.6) to the south 

end of Ridgely Yard (milepost 183.3) just north of Springfield, Illinois (milepost 182.0). 

The NAJPTC line operates under centralized traffic control (CTC) and follows the 

General Code of Operating Rules (known as GCOR). The line consists of single track 

with sidings for meets and passes as well as spurs and industrial sidings. Numerous 

highway crossings exist on the line, most of which were equipped by the NAJPTC 

project for radio-based advance activation to accommodate high-speed passenger trains 

5 
without having to extend detection circuits. Trains operating on this line are UP freight 

trains (through and local) and Amtrak passenger trains. The line included interlockings at 

crossings with foreign railroads. The NAJPTC system was designed to operate under 

these conditions. 

NAJPTC Line 

Figure 1. Testing of NAJPTC was 

in the State of Illinois 

The 120 miles of PTC-controlled territory was divided into two categories of roughly 60-

miles each: PTC integrated territory and PTC standalone territory. PTC standalone 

territory was intended to demonstrate the capacity improvements achievable with moving 

block operation. In addition to the incremental moving block authority limits (applicable 

to PTC trains only), trains on PTC integrated territory were additionally constrained by 

conventional fixed signals. Consequently, PTC integrated territory would not show 

capacity improvements, but was to demonstrate efficient handling of unequipped trains 

mixed with PTC-equipped trains. 

A third segment of track, the territory between Chicago and Mazonia, was known as 

PTC-monitored territory. In this area, PTC trains were monitored only for location and 

speed. 

The key elements of the communication-based train control architecture of the NAJPTC 

system include: 

• Mobile data radio, onboard computer, onboard display, locomotive interface, 

and location determination on each locomotive, 

6 
• Server in the dispatch center, 

• Wayside interface units at signals, switches, train defect detectors, railroad 

crossings, and highway crossings, and 

• Roadway worker terminal on roadway vehicles. 

Figure 2 shows each of the subsystems and their main functions. PTC functions are 

modular to allow tailored implementation for each railroad or territory. Functionality 

negotiation capabilities permit trains to automatically adjust to the specifics of each 

deployment infrastructure. This, along with an open architecture, facilitates 

interoperability (operating one railroad’s locomotive on another railroad’s track under 

PTC) and migration (supporting different levels of functionality at various stages of 

evolutionary implementation). 

Figure 2. PTC System Architecture 

The PTC system architecture is based on the moving block concept (whether integrated 

with fixed block signals as in integrated territory or operating in standalone territory). 

This means that a movement authority for a train can be issued between any two 

unoccupied track locations, rather than being constrained to fixed block boundaries as 

with conventional signaling. As Figure 3 shows, this has the potential to allow train 

spacing (headway) to be reduced and track capacity to be increased in following move 

(fleeting) scenarios and certain bottleneck situations, such as where track has been 

downgraded or returned to service after maintenance. 

7 
Appendix A contains photographs of the NAJPTC system elements along with a legend 

for the onboard display. 

Direction of Movement

4-Aspect

Signaling

Fixed

Block

Moving

Block

= Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Figure 3. Moving Block has the Potential to Reduce Headways 

NAJPTC system development followed a phased plan: 

Build 1: 

• Office, onboard, and communications infrastructure, and 

• Location determination, reporting, and tracking. 

Build 2A, a test build including all Build 1 functionality plus: 

• PTC integrated mode operation at 79 mph, 

• Track bulletin conditions, 

• Authority management in NAJPTC—integrated territory, and 

• Predictive and reactive enforcement. 

Build 2B for revenue service operation including all Build 2A functionality plus: 

• Implementation in NAJPTC—standalone territory, 

• Crossing advance activation, 

• High-speed passenger train operation (increase from 79 mph to 110 mph), and 

• Operation with upgraded RF communications for reduced fail-safe latency 

and higher throughput. 

Build 3: 

• Integration of high-speed defect detectors, 

• Remote entry of roadway worker requests, 

Excess Train

Separation

Braking Distance

plus Margin

Direction of Movement

4-Aspect

Signaling

Fixed

Block

Moving

Block

= Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Braking Distance

plus Margin

8 
• Pacing implementation and testing, 

• Emergency enforcement braking implementation and testing, 

• Cab signal interoperability demo, 

• Confirmed consist approach, 

• Functional negotiation capability testing, 

• Display of predictors, and 

• Online track database update capability. 

The NAJPTC project was terminated before the completion of Build 2A because IDOT 

needed to begin high-speed operation sooner than would be achievable, given the 

difficult PTC design challenges remaining to be solved. Consequently, Builds 2B and 3 

were not fully implemented and tested, although requirements and much of the design 

was done. See Appendix B for further details of NAJPTC functionality that was 

implemented and tested. 

9 
2.0 Definitions 

The terms and definitions listed below are specific to the NAJPTC project and may or 

may not conform to definitions of similar terms used in other contexts. 

Automatic 

Equipment 

Identification 

An automated scanning and tracking system that reads radio 

frequency tags mounted on locomotives and rolling stock using 

wayside scanners. 

Automatic Train 

Control 

A system to enforce compliance with cab and wayside signal 

indications. If the train exceeds a predetermined speed for a given 

signal indication and speed is not reduced at a sufficient rate, 

brakes are automatically applied. 

Block 

A length of track: 

• between consecutive block signals, 

• between a block signal and the end of block system limits, or 

• in Automatic Train Conrol, limits the use of which is governed 

by cab signals and/or block signals. 

Block Signal A fixed signal at the entrance of a block that governs trains 

entering and using that block. 

Braking Algorithm 

The algorithm used to predict a train’s braking performance, based 

on such factors as train length, train weight, braking characteristics, 

speed, and grade. 

Bulletin 

Information concerning speed restrictions, work limits, or other 

instructions relayed to train crews. In General Code of Operating 

Rules (GCOR), these include Form A, Form B, and Form C 

bulletins. 

Busy Bit 

Link access protocol similar to carrier sense multiple access 

(CSMA) but for duplex channels. Base stations transmit bits 

indicating whether or not the inbound channel is busy. A node 

wanting to transmit to that base station uses this information in 

determining when to transmit. 

Centralized Traffic 

Control 

A block system that uses block signal indications to authorize train 

movements. 

Communicating 

Train 

A train that includes a controlling locomotive equipped with PTC 

equipment that is functional in communication with the PTC 

communications network. Where the word train is used in a 

requirement, it is assumed to be a communicating train unless it is 

specifically described as noncommunicating (see 

Noncommunicating Train below). 

Computer-Aided 

Dispatch 

A system that provides rail dispatchers with a dynamic display of 

territory and may automate features such as meets and passes, train 

tracking, and routing. 

Configuration 

Management 

A process that addresses revision control measures for products, 

material modification of hardware and software, and methodology 

to demonstrate that the proper and intended product configuration, 

including the actual functional and physical characteristics, are 

maintained. 

10 
Confirmed Train A train whose length and weight have been verified in a vital 

manner. 

Consist The set of cars and locomotives comprising a train. 

Control Point The location of absolute signals controlled by a control operator. 

Distributed Power 

The practice of placing locomotives, which can be remotely 

controlled in the middle or rear of the train, to improve train 

handling. 

Employee-inCharge 

Refers to supervisor of roadway workers working on or about the 

right-of-way. As used in this document, it refers to the authorized 

user of the Roadway Worker Terminal (RWT). 

Enforcement 

The act of applying train brakes automatically and safely to keep 

the train in compliance with the constraints of allowed speed, track 

occupancy, authority limits, and direction of travel imposed by a 

control system. 

End-of-Train 

device 

A device that monitor’s the train’s air brake system and train 

integrity. It has a flashing red light (at night) and provides the 

capability to make an emergency brake application from the rear of 

the train. 

Engineer The qualified and responsible operator of a locomotive. 

Fail Safe 

A design philosophy applied to safety-critical systems such that the 

result of hardware failure or the effect of software error shall either 

prohibit the system from assuming or maintaining an unsafe state, 

or shall cause the system to assume a state known to be safe (IEEE 

1483). 

Fixed Block 

Operation 

A method of operation where authorities are issued to fixed 

wayside points, such as mileposts, switches, roadway signal 

locations, and stations. 

Full Service 

Application 

An application of the brakes resulting from a continuous or split 

reduction in brake pipe pressure at a service rate until brake 

cylinder pressure is equalized with auxiliary reservoir pressure. 

Functionality 

Negotiation 

The process of identifying the highest level of common 

functionality that can be achieved safely by onboard and off-board 

PTC systems. 

General Code of 

Operating Rules 

A set of operating rules used by a number of North American 

railroads. These rules are meant to promote safety, and cover topics 

including general employee responsibilities, signals and their use, 

and procedures for the safe movement of trains. 

High AccuracyNationwide 

Differential Global 

Positioning System 

An upgrade to differential global positioning that provides the 

capability to broadcast corrections to the global positioning system 

(GPS) over long ranges to achieve a better than 10 centimeter (cm) 

(95 percent) accuracy throughout the coverage area. 

Location 

Determination 

System 

The system onboard the locomotive that determines the train’s 

location and speed. 

11 
Management 

Information 

System 

The component of the railroad’s information system that maintains 

data on train consists, schedules and may also provide the means 

for issuing bulletins. 

Movement 

Authority 

Permission given to a train crew or work crew to occupy a track 

and move within defined limits. 

Moving Block 

Operation 

A method of operation in which authorities are issued to the rear of 

a preceding communicating train in following move operations 

instead of to fixed block boundary locations. 

Noncommunicating 

Train 

A train that that does not include a PTC-controlling locomotive or 

one that includes a PTC-controlling locomotive equipped with PTC 

onboard equipment that has failed or is not in communication with 

the PTC communications network. 

A railroad office location from which rail operations are controlled 

and monitored. An office may control a whole railroad or a single 

division. An office may be completely manual using pen, paper, 

and voice communications, or may be highly automated with 

sophisticated computer support. 

Office 

The act of reporting the arrival, departure, or passing of a train at a 

specific location. On Sheet 

The set of rules specifying the operation of a railroad, which are 

meant to promote safety, and cover topics including general 

employee responsibilities, signals and their use, and procedures for 

the safe movement of trains. 

Operating Rules 

Positive Train 

Control 

A train control system that meets the three safety objectives 

defined by the Railroad Safety Advisory Committee: 

Prevention of train-to-train collisions, prevention of derailments 

due to train operation in excess of permitted speeds, and protection 

of roadway workers operating within the limits of their authorities. 

Predictive 

Enforcement 

The action of enforcing a train to prevent a predicted violation of 

authority, restriction, or speed limit that would otherwise occur. 

Product Safety Plan 

A detailed description of a PTC system and its operation, including 

hazard logs, human factors analysis, and risk assessment. A PSP is 

required to be submitted to FRA for approval to allow a PTC 

system to be operated in revenue service. 

Reactive 

Enforcement 

The action of enforcing a train after violation of the limits of its 

authority, restriction, or speed limits. 

Restricted Speed 

A train operating speed that allows the locomotive crew to stop the 

train within half of the range of vision, short of a train, engine, 

men, or equipment fouling track, stop signal, derail, or improperly 

lined switch, and looking out for broken rail, not to exceed the 

maximum speed defined by the rulebook in force. A typical 

maximum speed allowed under restricted speed authority is 20 

mph. 

12 
Reverse Move 

A movement by the locomotive or portion of a train in the opposite 

direction of its unidirectional authority that results in movement of 

the rear of the train in the direction opposite of the authority. An 

example of a reverse move is a train that is moving its entire 

consist in reverse. 

Risk An expression of the possibility/impact of a mishap in terms of 

hazard severity and hazard probability. (Military Standard 882C) 

Risk Analysis 

The process determining, either quantitatively or qualitatively, the 

measure of risk associated with (1) use of the product under all 

intended operating conditions or (2) the previous condition. (49 

CFR 236 Subpart H) 

Roadway Workers Maintenance and inspection personnel that require access to areas 

of the railroad including track to perform their assigned duties. 

Safety Critical 

A designation applied to a function, a system, or any portion 

thereof, the correct performance of which is essential to safety of 

personnel and/or equipment, or the incorrect performance of which 

could cause a hazardous condition, or allow a hazardous condition, 

which was intended to be prevented by the function or system, to 

exist. (49CFR236 Subpart H) 

Siding 

A section of track that leaves a main line and enters back onto it 

(most often long enough to fit an entire train), which allows one 

train to move aside while another passes. 

Signal Block See Block 

Speed Restriction 

A speed limit that may be permanent or temporary for a certain 

location or type of train on the basis of direction of travel or track 

on which the train is operating. 

Temporary Speed 

Restriction 

A bulletin that prescribes a speed restriction over track within 

certain limits. Temporary speed restrictions are applicable to the 

entire length of a train, unless qualified as “head-end only” or “stop 

and inspect.” 

Track Circuit A circuit that allows for the detection of train occupancy within a 

section of track. 

Track 

Discrimination 

Determining which track a train is on when it is within multipletrack territory. 

Train One or more locomotives carrying a marker with or without cars. 

A transportation research, testing, and consulting company 

providing emerging technology solutions for the railway industry 

throughout North America and the world (a wholly owned 

subsidiary of the Association of American Railroads). 

Transportation 

Technology Center, 

Inc. 

A function in a safety-critical system that is required to be 

implemented in a fail-safe manner. (IEEE 1483) Vital function 

Wayside Interface 

Unit 

A device that provides remote monitoring and control of wayside 

devices. 

Work Limits The locations between which a work authority has been issued. 

13 
3.0 Benefits from the NAJPTC Project 

In addition to the safety objectives listed in Subsection 1.1, the NAJPTC system was 

designed to accommodate 110-mph passenger trains. In addition, vital, moving block 

(standalone) PTC has the potential to increase railroad capacity by reducing excess 

headway as compared with fixed block signaling and by reducing congestion via pacing. 

It also has the potential to decrease train control system life-cycle cost by reducing the 

amount of wayside vital equipment required. 

3.1 Potential Headway and Capacity Benefits of Vital, Moving Block PTC 

Although development of the standalone mode (i.e., vital, moving block PTC) was a 

Build 2B work item that was not completed before the project was terminated, the design 

progressed far enough to quantify the standalone mode’s parameters and braking 

algorithm margins. This in turn allows projection of the potential reduction in headway 

and potential increases in capacity of the NAJPTC moving block implementation as 

compared with a conventional 4-aspect signaling system. Since Build 2B was not 

completed, the potential benefits of moving block have not been proven by actual 

implementation and field testing. 

The potential headway reductions and potential capacity increases to be gained by using a 

vital, moving block PTC system were analyzed with a variable warning time to the 

engineer (indicating that braking enforcement was imminent) for a loaded 123-car grain 

train considering various block lengths, warning times, grades, and brake algorithm 

margins. Trains were modeled operating in a following move scenario at the same speed 

(e.g., double track current-of-traffic, or fleeting on single track). 

The mode of operation for use in NAJPTC Standalone Territory was a hybrid between 

fixed and moving block train control. The reason for this hybrid configuration is that two 

enabling technologies were unavailable that are necessary to obtain significant benefits 

from moving block at speeds above 49 mph. The two enabling technologies are (1) a 

vital train consist determination and integrity monitoring system (to detect pull-aparts) 

and (2) a broken rail detection system that does not impose traditional fixed block train 

separations (headways). In the absence of these two enabling technologies, the NAJPTC 

project depended on conventional track circuits (associated with the signaling system) to 

address train consist/integrity and broken rail detection in standalone mode (as well as 

integrated mode). To achieve the maximum potential headway reduction from moving 

block operation, a more accurate brake-distance prediction algorithm would also be 

required. 

As illustrated in Figure 4, NAJPTC’s hybrid moving block/fixed block concept enforced 

a 20 mph speed restriction (upper limit of a restricted speed restriction) through an 

intermediate block that was occupied or that contained a broken rail, rather than 

enforcing an authority limit (stop) at the last reported rear end location of the leading 

train. This dependency on track circuits significantly limited the potential capacity or 

headway improvement otherwise attainable from moving block operation. Figure 5 

shows the degree of performance improvement achievable with the hybrid moving 

block/fixed block mode of operation. 

14 
Direction of Movement

4-Aspect

Signaling

Fixed Block

Hybrid 

Moving Block/Fixed Block

Speed

•Signal Tumble-Down Time*

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

Safety Buffer

Braking Curve

•Signal Sighting Time* •Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

Up to 1 block

60 mph

20 mph

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Direction of Movement

4-Aspect

Signaling

Fixed Block

Hybrid 

Moving Block/Fixed Block

Speed

•Signal Tumble-Down Time*

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

Safety Buffer

Braking Curve

•Signal Sighting Time* •Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

Up to 1 block

60 mph

20 mph

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Figure 4. Relationship of Moving Block and Conventional Signaling Parameters 

Used in Analysis of Potential Headway Reduction and Capacity Increase for a 

Hybrid Moving Block/Fixed Block PTC System 

The standalone mode of NAJPTC could also be configured to operate in pure moving 

block mode, i.e., where train separation is governed primarily by safe braking distance to 

the train ahead without any constraints from a signaling system. This mode of operation 

offers much greater potential headway and capacity benefits but requires two currently 

unavailable enabling technologies (mentioned earlier), or else it can only be used at 

speeds below 50 mph. 

For pure moving block operation, the performance was modeled for the same loaded 123-

car grain train traveling 60 mph on a level route, using a block length of 2.5 miles, a 20-

second warning time, and the NAJPTC brake algorithm margin. 

15 
0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec 

 Enforced Speed Restriction: Yes

Potential Headway Reduction (sec)

114

227

341

455

568

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: Yes

4X

3X

2X

 

Figure 5. Potential Headway Reduction and Capacity Increase for a Hybrid Moving 

Block/Fixed Block PTC System vs. 4-Aspect Signaling for a Typical Grain Train

The analysis shows that a vital, moving PTC system could provide a headway reduction 

of 27,745 feet, or 315 seconds, between trains for this scenario, corresponding to a 

potential capacity increase of 133 percent (where a capacity increase of 100 percent 

corresponds to a doubling of capacity) when compared to a conventional, 4-aspect 

signaling system using a block length of 2.5 miles. This amount of capacity increase is 

only achievable in scenarios where trains operate in close following moves, such as 

double track or fleeting. In single track, nonfleeting scenarios, where meets occur 

frequently, the potential capacity improvements from moving block PTC are not as 

significant. 

The potential savings in headway and increase in capacity can be quantified for this same 

scenario considering other block lengths. Figure 6 shows the results for block lengths 

ranging from 1.25 to 2.5 miles (in increments of a quarter mile). 

16 
0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

114

227

341

455

568

0

 

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

-3x 

-2x 

-4x 

Figure 6. Potential Headway Reduction and Capacity Increase for a Vital, Moving 

Block PTC System vs. a 4-Aspect Signaling System for a Typical Grain Train 

The results show that in spite of the significant conservatism (margin) that was designed 

into the NAJPTC braking algorithm (to allow for uncertainties in train characteristics), 

the potential capacity increases, or headway reductions are significant. Appendix C 

includes results for additional scenarios comparing performance under moving block 

versus conventional train control. 

It should be noted that while moving block (“standalone”) PTC has the potential to 

increase railway capacity and reduce headways, the integrated mode (or an overlay PTC 

system) cannot increase capacity—it can only degrade capacity and headway (due to 

braking algorithm margin and system delays), because it imposes additional constraints 

beyond those imposed by the conventional signaling system. This degradation is 

expected to diminish, however, as accuracy of predicting braking distance improves by 

future development. 

17 
3.2 Additional Benefits Provided by Vital, Moving Block PTC 

Vital, moving block PTC has the potential to provide benefits in addition to those already 

cited. 

One of these potential benefits is faster recovery from track outages or temporary speed 

restrictions. With vital, moving block PTC, there would be no holding back a following 

train at an absolute signal while waiting for the entire block to be cleared by the leading 

train. Instead, the following train could follow the leading train at safe braking distance. 

Additionally, no signal-imposed speed restriction (e.g., restricted speed) for trains 

following within the same block would occur. 

Another potential benefit provided by vital, moving block PTC would be that there would 

be less time lost during overtake or pass situations. When a faster train is following, the 

closing up process when entering the meet could be tighter, and the slower train would be 

able to depart from the siding sooner after the pass. 

Installing a vital, moving block PTC would also eliminate the need for wayside signals 

and their associated cost and maintenance. Where broken rail detection would not be 

required or where alternatives exist, no need for track circuits would be required. 

Consequently, reduction or elimination of costly vital wayside equipment is anticipated 

as a potential long-term benefit when vital, moving block (standalone) PTC is fully 

deployed on a corridor. 

3.3 Benefits Derived from the NAJPTC Project 

In addition to the lessons learned (addressed later), the benefits achieved by the NAJPTC 

project include: 

• By integrating inertial sensors with DGPS, tachometer readings, and map 

matching, NAJPTC’s onboard location determination system (LDS) has 

increased the ability to dead reckon in areas lacking satellite coverage (such as 

in tunnels or “urban canyons”), significantly improved track resolution ability, 

and improved the detection of failure modes over systems that do not 

incorporate inertial sensors. 

• Portions of the NAJPTC system’s hardware and software implementation 

have been reused by other PTC projects. 

• Open specifications/design documentation from the NAJPTC project has been 

used in other PTC projects. 

• Being the first to attempt many new train control approaches, NAJPTC testing 

in Illinois identified a number of implementation issues from which valuable 

lessons have been learned and solutions developed, benefiting other PTC 

projects (see Section 5.0). 

18 
• The NAJPTC system makes greater use of commercial, off-the-shelf hardware 

and software components than did prior vital train control systems, paving the 

way for lower life cycle costs. 

• First freight train control system to attempt vital tracking of non-PTC trains 

(integrated with PTC trains) via track circuits from which valuable lessons 

were learned and are addressed in this report. 

• First project to apply new FRA rule (49 CFR, Part 236, Subpart H), resulting 

in a better understanding of what is required to apply it to an actual 

development project. 

• Safety case (product safety plan, or PSP) provided a template and source 

material for other PTC projects. 

19 
4.0 Design Challenges 

Many challenges have existed and will continue in the development, refinement, and 

testing of PTC. The following are among the most challenging issues that have been 

encountered so far in its development: 

• How to determine train location and make turnout decisions (track 

discrimination) with safety-critical dependability, 

• How to predict enforcement braking distance accurately enough that a train 

will not go past the target nor be stopped far short of the target, 

• How to verify train consist characteristics, and 

• How to meet system performance requirements given the performance 

limitations of mobile radio communications. 

The first three items above are particularly difficult to accomplish with the necessary 

degree of integrity required for vital train control. The potential solutions to challenging 

problems like these often have associated life-cycle costs and/or operational implications. 

Therefore, the railroad(s), system engineer, and system developer must carefully analyze 

and select the most appropriate solution. 

Further descriptions of the above issues and others along with the approaches selected for 

the NAJPTC project are presented below. 

4.1 Location Determination, Track Discrimination, and Entry Point Protection 

4.1.1 The Challenge 

A fundamental concept common to PTC systems is that trains must continually and 

accurately determine their location on board and transmit it to an off-board server rather 

than having their location determined coarsely by occasional wayside devices such as 

track circuits or axle counters. Without accurate train location determination in a PTC 

system, uncertainty buffer distances must be increased, resulting in productivity impacts 

including increased train headways, less usable siding capacity, and stopping trains 

shorter of signals than necessary. 

In addition, it is extremely critical to know which track each train is occupying in areas of 

parallel track. The function of identifying the correct track is referred to as “track 

discrimination.” The challenge in train location determination and track discrimination is 

in meeting the very high-integrity requirements required for safety. Without highintegrity track discrimination, a PTC system must depend upon human input, which 

increases safety risk. Additional ramifications of various solutions are addressed below. 

4.1.2 Potential Solutions 

An obvious solution to the location determination and track discrimination problem 

might seem to be installation of a differential global positioning satellite (DGPS) receiver 

on board each locomotive. DGPS can provide very high-integrity location determination 

20 
based on multiple satellites with accuracy on the order of that needed to meet the PTC 

requirement for along-track accuracy of better than 10 ft with 99 percent confidence. 

However, DGPS by itself is not accurate enough for high-integrity track discrimination of 

the nature required for a vital train control system. Specifically, the probability of correct 

track discrimination where parallel tracks may be 11.5 ft apart is less than 0.99 with 

ordinary DGPS alone; whereas, the requirement for train control is on the order of 

0.999999. Also problematic is when DGPS coverage is temporarily lost, most often 

because of signal blockage in certain locations. 

Monitoring all switches and tracking train movements through switches can solve the 

track discrimination problem with DGPS. With this approach, however, the problem of 

DGPS blockage remains. This problem can be addressed to some extent by using 

locomotive tachometer information to allow the LDS to coast (dead reckon) through 

temporary DGPS signal outages. However, this solution only works for limited DGPS 

signal outages because tachometer distance error accumulates with wheel slip and creep. 

Although the combination of DGPS, tachometer, track database, and switch monitoring 

may work sufficiently for non-vital overlay PTC systems and for vital systems over a 

limited corridor, more than 50,000 miles of track in the United States do not have 

monitored switches; economics generally will not justify equipping all of them with 

monitored switches solely to support PTC. 

The solution used in the European Train Control System is to install transponder tags at 

strategic track locations (e.g., on each leg of a turnout) and to have a tag reader on board 

each locomotive used in conjunction with the tachometer. This approach can perform 

reliable track discrimination and does not suffer from signal blockage of the DGPS 

nature. Whereas this solution is practical in some scenarios, it is not considered practical 

for general use on U.S. freight railroads, which operate over 95,000 miles of track. The 

issues have to do with the costs and logistics of installing and maintaining tags, the 

configuration management associated with their use (e.g., ensuring that tags with a 

specific serial number are located correctly), and preventing their vandalism on many 

miles of track, as well as maintaining tag readers underneath locomotives. 

Whether a satellite or transponder-based approach is used, knowledge of switch position 

is needed for two purposes: (1) for display to the train crew on board and (2) for 

enforcement of authorities and speed restrictions. Consequently, a need exists to monitor 

turnouts vitally (either directly or through detection of track circuit shunting) in areas 

where train speed can be high (e.g., over 49 mph) or density can be high, regardless of 

the type of locomotive-based LDS employed. 

4.1.3 NAJPTC Approach and Rationale 

The NAJPTC LDS approach uses the combination of technologies illustrated in Figure 7, 

which has been selected to address the above issues. 

21 
Figure 7. NAJPTC Onboard Location Determination System 

NAJPTC’s multiple-sensor, inertial navigation system (INS) design handles the various 

LDS challenges described above and performs self-checking to maintain safety. For 

example, when temporarily out of DGPS signal coverage, the INS components allow the 

system to continue operating. The gyro provides extremely precise turn rate information 

to allow dependable turnout decisions (track discrimination) as a locomotive passes 

through a facing-point turnout. This compensates for the insufficient accuracy of DGPS 

alone to reliably perform track discrimination. The gyro also helps to correct tachometer 

errors (due to wheel slip and creep) as the train passes through curves. This is useful in 

areas lacking DGPS coverage, such as in tunnels and urban canyons. The accelerometers 

also help in verifying turnout decisions as well as determining direction of movement 

(forward versus reverse), which most tachometers are not designed to detect. 

Additionally, for the NAJPTC implementation in Illinois, all turnouts are monitored 

because of the moderate and high-speed operations on that territory. 

One of the more challenging requirements of the NAJPTC system was that it prevents 

unauthorized trains from entering PTC territory, using brake enforcement if necessary. In 

order for the NAJPTC system to enforce, it first had to be enabled, and to enable, it 

needed to know which track the train occupied (track discrimination must have occurred). 

This could be technically very difficult for trains initializing outside of PTC territory, due 

to the lack of prior continuous location tracking history up to that point. The pre-enable 

track discrimination problem had to be solved in each of three different scenarios 

(referred to as “POD 1”, “POD 2”, and “POD 3”, where POD stands for “Point Of 

Discrimination”). The POD 1 scenario was the easiest to accommodate. In POD 1, no 

22 
other tracks were within 50 feet of the entry track, so the LDS could definitively resolve 

track based on GPS and the track database alone. If, however, a train initializes on one of 

multiple closely spaced tracks, onboard LDS systems generally are not able to verify 

which track the train is occupying with a high enough level of safety integrity until the 

locomotive moves through a track feature with a LDS-detectable signature profile (e.g., a 

turnout or a unique curve). NAJPTC correlates location reports with track circuit 

occupancy information when available and sufficient to resolve this initialization 

ambiguity (POD 2). Otherwise, the ambiguity in this POD 3 scenario would not be 

resolved until alternative methods are used, such as moving the locomotive through an 

unambiguous track feature, operator input is obtained, or the system design is modified as 

described below. The solution for POD 3 scenarios was not resolved or implemented as 

it was a Build 3 function. 

The track discrimination problem at initialization can be further (but not totally) 

mitigated by having the LDS on board every PTC locomotive enabled and tracking at all 

times when the locomotive has power available, regardless of whether or not the PTC 

system has been enabled and initialized. This also requires use of the onboard data radio 

to verify that the locomotive’s track database is current at all times. This potential POD 3 

approach was considered for Build 3. High accuracy nationwide differential global 

positioning system (HANDGPS), if and when it becomes widely available, would offer a 

better potential solution, because it would eliminate the need to sometimes rely on human 

input or passage through a suitable track feature before being able to provide PTC 

protection to a train. 

During Build 1 field testing, NAJPTC’s onboard LDS was shown to meet system 

performance requirements at speeds ranging from 2 to 110 mph. It consistently made 

correct turnout decisions well before the train reached the frog of the turnout. 

Throughout both phases of the project, the LDS consistently performed very well. 

4.2 Consist Determination and Verification 

4.2.1 The Challenge 

PTC requires dependable consist (train makeup) data, particularly train length and 

weight, to support the following safety critical functions: 

• Rolling up movement authority limits in Standalone Territory (moving block), 

• Enforcing movement authority limits during reverse moves, 

• Predicting braking distance (for enforcement and warnings), 

• Predicting future train location (for onboard display and activation of highway 

crossings), and 

• Displaying train length on board. 

23 
Although railroads generally have data on every consist, that data may not be dependable 

and/or timely enough for use in safety critical train control functions. Thus, additional 

measures need to be taken. 

A related issue is that after consist data has been determined, the system must monitor 

train integrity to know if and when the consist may have changed. 

Besides the safety risk associated with non-fail-safe detection of pull-aparts in unsignaled 

territory, there are productivity ramifications of inaccurate consist determination. These 

stem from having to assume worst case train consist characteristics that result in 

increased train headways, less usable siding capacity, and stopping trains shorter of 

signals than necessary. 

4.2.2 Potential Solutions 

The potential solutions are based on the possible sources of consist information. Railroad 

management information systems (MIS) and/or computer-aided dispatch (CAD) systems 

generally have data on consists. This data is not intended for vital use, so it is not created 

or stored with the level of integrity necessary for fail-safe applications. Because consist 

data is provided to PTC from the CAD system (whether originating at CAD or MIS), it is 

referred to as CAD-supplied. Train crews also have consist information (from the same 

sources) but it is not generally adequate for train control purposes. 

There are a number of wayside devices that can provide various consist information. 

These include: 

• Automatic equipment identification (known as AEI), 

• Hot bearing detectors (HBD), 

• Wheel impact load detectors (WILD), 

• Track circuits. 

Today, other than track circuits, not nearly enough of these devices are deployed along 

U.S. railroads to meet the consist data needs of PTC. Although track circuits have been 

traditionally used to address these same fundamental issues, they are quite expensive to 

maintain, and they negate the potential benefits of moving block. Consequently, the 

elimination of track circuits is a long-term objective that was beyond the scope of the 

NAJPTC project. An alternative, less expensive, means of mitigating broken rail risk 

(e.g., detecting rail breaks) to support reduced headways is a key enabler that is not 

available yet. On most U.S. railroads, the train defects detectors only transmit a 

synthesized analog voice report to the train over the voice radio. 

Another solution that has been proposed is an enhanced end-of-train (EOT) device that 

would determine train length or location. Some suppliers have added a GPS to an EOT 

device, but these devices have not been validated for vital PTC applications. Design 

challenges exist to achieving adequate satellite coverage in the EOT environment (i.e., 

24 
hemispherical blockage by end of railcar). Also, coupler mounting conditions are much 

less than rigid given the need for quick, easy connection and disconnection. Moreover, 

the logistics associated with introducing and maintaining a second type of EOT device 

into railroad fleets are a deterrent. 

An alternative approach is to include a locomotive (equipped with GPS and a data radio 

to detect and report end-of-train location to the PTC equipment at the front of the train) at 

the end of each PTC train under the control of distributed power (DP). The rear 

locomotive can also help manage in-train forces while reducing the length and variance 

of stopping distance and detecting/reporting pull-aparts. GPS on the rear locomotive can 

be used to determine rear-of-train location and to compute train length. The economic 

justification for including a locomotive at the rear of each PTC train would be further 

increased if that locomotive hosted a broken rail detector. 

Derivation of consist data from onboard measurements of train acceleration and/or 

deceleration characteristics has also been considered. Based on ΣF=MA (sum of the 

forces equals mass times acceleration) calculations, train weight can be calculated if a 

train’s total horsepower is accurately known. Braking distance estimates for predictive 

enforcement (one of the key products of consist data) can be verified from samples of 

routine air brake applications while moving en route. Unfortunately, many trains go a 

long distance, possibly an entire trip, without applying the air brakes enough to obtain 

data of relevance to braking distance estimation. Other measures can be taken to improve 

the accuracy of braking prediction algorithms and to make them more adaptive, including 

measuring the propagation time of induced brake pipe pressure changes at the EOT 

during brake application and monitoring the braking efficiency of the train. 

An indication of train integrity can be inferred by monitoring rear-of-train brake pipe 

pressure using standard EOT devices. But not all passenger trains in the United States 

are equipped with EOTs. More modern passenger trains typically have an electric 

trainline running through the cars and locomotives that may be able to provide train 

integrity information, but it does not always run the entire length of the train, particularly 

when it is made up of newer and older cars. 

In dark territory situations where a maximum freight train speed of 49 mph (59 mph for 

passenger trains) is acceptable, moving block operation may be viable without the need 

for vital equipment to detect train integrity, train presence, and broken rails (i.e., without 

track circuits, vital EOTs, etc.), since they’re not generally required today (per federal 

regulations) for operation at these speeds. However, traffic density may be a 

consideration, per 49CF236 subpart H. 

4.2.3 NAJPTC Approach and Rationale 

Because of the wide potential variations and uncertainties in freight car consist 

characteristics, the consist verification challenge lies primarily with freight trains. The 

NAJPTC system design addresses the problem by using two consist modes: 

“Unconfirmed” and “Confirmed” . When a train is initialized, it enters Unconfirmed 

Consist mode. If and when sufficient data becomes available for NAJPTC to verify the 

25 
consist characteristics with integrity adequate for safety critical purposes, it transitions to 

Confirmed Consist mode. 

In Unconfirmed Consist mode, several conservative measures are taken for safety 

reasons. Due to potential wrong-side errors (unsafe failure modes), the predictor, which 

shows the crew the expected location and speed of their train 30 s and 1 min into the 

future, is not displayed. For similar reasons, the train length is not displayed when the 

consist is unconfirmed. Until the consist is confirmed, train weight is established as the 

heavier of (a) the worst-case weight of all train types, and (b) consist weight provided by 

the CAD system. Authority roll-up behind a train is based on track circuit occupancy 

indications while in Unconfirmed Consist mode. So, no moving block is behind a train in 

Unconfirmed Consist mode. This protects the rear end of the train. 

A train transitions from Unconfirmed Consist to Confirmed Consist mode if and when its 

length and weight are measured and found to match the CAD-supplied consist length. If 

they never match, the train never leaves Unconfirmed Consist mode, thus requiring that 

the most conservative parameter values continue to be assumed for that train’s consist. 

Train length is measured by correlating occupancy and clearance reports from a 

monitored track circuit at a control point (referred to as an OS) with time-stamped 

location reports from the train. The method of measuring weight may be based on 

monitoring acceleration performance and performing ΣF=MA calculations given the total 

locomotive horsepower, which train crews can confirm. 

Once in Confirmed Consist mode, some of the extremely conservative measures applied 

in Unconfirmed Consist mode are no longer required. Train length and predictors are 

displayed; authority roll-up is based on confirmed train length, so moving block 

operation can now occur in Standalone Territory. However, even in Confirmed Consist 

mode, the variations in train-braking characteristics (e.g., due to train weight uncertainty, 

brake-rigging efficiency uncertainty, valve type, etc.) are significant. So, a significant 

amount of margin is still needed when predicting braking distance. 

A train transitions from Confirmed Consist back to Unconfirmed Consist mode upon 

occurrence of certain “consist events.” Sample consist events are: 

• Pull-apart, as detected by a loss of rear brake pipe pressure or as entered by 

the crew, 

• Discrepancy detected between CAD-supplied consist length and that obtained 

from track circuits in conjunction with location reports (due to an update from 

either source), and 

• Train stops (this includes when work activities are performed). 

For the NAJPTC implementation in Illinois, track circuits remain in place throughout 

both Integrated and Standalone Territory. This allows the moving block train-integrity 

problem to be solved by a combination of the following four methods: 

26 
1. A speed restriction is imposed behind a train to the end of the block it 

occupies. The value of this speed restriction is configurable and can be 

adjusted based on risk analysis. 

2. Rear of train brake-pipe pressure is monitored for unexpected loss of pressure. 

3. Train crews can indicate a train separation to NAJPTC via the onboard HMI. 

4. Train length is measured every time a train occupies and then clears an OS. 

Verifying train length and weight to transition to Confirmed Consist mode was planned 

for implementation in Build 3 of the project. Since Build 3 was not initiated, the 

NAJPTC system always operated in “Unconfirmed Consist” mode for freight trains. 

Four worst-case consists were provided for passenger trains, and the train crew was 

required to select the appropriate one, from the following types: 

• Train Type A (maximum speed of 110 mph)—a future high-speed trainset 

(consist to be determined), 

• Train Type B (maximum speed of 110 mph)—a defined combination of 

Amtrak locomotive(s) and Amfleet, or Horizon cars, baggage cars, or 

RoadRailer vans, 

• Train Type C (maximum speed of 90 mph)—a defined combination of 

Amtrak locomotives, and Amfleet, Horizon, or Superliner cars, baggage cars, 

or express cars, including RoadRailers, and 

• Train Type D (maximum speed of 90 mph)—all Amtrak trains that do not 

qualify as Train Type A, B, or C. 

The passenger enforcement algorithms used fixed enforcement equations that were based 

on train type, but independent of train size within the train type. The freight enforcement 

algorithm computed braking distance using a worst-case consist for Build 2 testing. 

Whether using worst-case consist or confirmed consist, the NAJPTC system specification 

required that freight trains stop short of the enforced target (not overrun) with 99.9995-

percent probability. This was probably an excessively conservative requirement. 

However, it has been adopted for most other PTC systems as well. 

Whatever probability is specified for enforcement target overrun, a PTC-braking 

algorithm must apply an offset (margin) to compensate for the unknown variations in 

train braking characteristics (e.g., due to unknowns in brake-rigging efficiency, brake 

valve type, coefficient of friction, train weight, number of operative brakes) to meet this 

requirement. In other words, the PTC system must apply the brakes earlier than required 

for a typical train. Figure 8 is an example of the amount of offset required. The example 

shown is for a grain train of 123 loaded cars, 4 locomotives with 12,000 horsepower total, 

6,798 feet long, and weight of 14,088 tons. The offset required in order to meet the 

27 
99.9995-percent requirement is 1,762 feet. In other words, the average train with the 

above stated characteristics would stop 1,762 feet short of an enforced target. 

Figure 8. Simulated Braking Distribution Shows that an Average Loaded Grain 

Train Running at 60 mph Would Be Stopped 1,762 feet Before Necessary by the 

PTC Braking Algorithm with “Confirmed” Consist 

Having PTC assume the worst-case consist at all times for freight train operation, 

however, would not be practical in revenue operation due to the excessive conservatism 

this would require in stopping trains (i.e., stopping trains too soon). This is apparent 

from Figure 9, which shows that 97.3 percent of the time PTC would stop a train 

operating at 10 to 30 mph more than 500 feet from the target using worst-case consist 

assumptions for the braking algorithm. Figure 10 shows that 100 percent of the time PTC 

would stop a train operating at 30 to 60 mph more than 1,000 feet from the target using 

worst-case consist assumptions for the braking algorithm. 

28 
Figure 9. With Worst-Case Consist Assumptions, No Overruns Occur, But a 10–30 

mph Train Stops within 500 ft of the Target only 2.7% of the Time 

Figure 10. With Worst-Case Consist Assumptions, No Overruns Occur, But a 30–

60 mph Train Never Stops within 1,000 ft of the Target 

The braking situation improves when a freight train transitions to Confirmed Consist 

mode but it would still impact productivity. This is apparent from Figure 11, which 

shows that 21 percent of the time PTC would stop a train operating at 10 to 30 mph more 

than 500 feet from the target using Confirmed Consist mode assumptions for the braking 

algorithm. Figure 12 shows that 62 percent of the time PTC would stop a train operating 

at 30 to 60 mph more than 1,000 feet from the target using Confirmed Consist 

assumptions for the braking algorithm. 

29 
Figure 11. With Confirmed-Consist Assumptions, No Overruns Occur, But a 10–

30-mph Train Stops within 500 ft of the Target only 79% of the Time 

Figure 12. With Confirmed-Consist Assumptions, No Overruns Occur, But a 30-60 

mph Train Stops within 1,000 ft of the Target only 38% of the Time 

Because of the significant negative impacts such conservative PTC braking algorithms 

would have on railroad operations and because this problem is universal to all PTC 

implementations to date, a separate project has been initiated under FRA sponsorship to 

develop a more accurate algorithm that would adapt by measuring the actual braking 

performance and related characteristics of each specific train. 

4.3 Mobile Radio Communications 

4.3.1 The Challenge 

In a moving block architecture, a train’s continued movement depends upon its receiving 

periodic movement authority updates as the track clears ahead. These movement 

30 
authorities can be much shorter than conventional signal blocks, and therefore must be 

updated frequently, particularly in the case of trains following one another at moderate or 

high speed. 

Because PTC is a CBTC system, train location messages are transmitted periodically via 

the radio communication system to the controlling PTC server. The server in turn 

transmits updated nonoverlapping movement authority messages to trains. In addition, 

numerous other types of messages are communicated among trains, wayside equipment, 

and the server. Figure 13 depicts key information flows within the PTC system. In 

addition, the PTC-specific data flows shown, some railroads share their train control data 

link with other applications such as code line and/or work order reporting. The NAJPTC 

line uses the train-control data link as the channel for conveying code line (CTC signal 

code) messages. 

Figure 13. Key Information Flows in PTC (Note: In addition to the information 

messages shown, other messages such as heartbeats and acknowledgements can add 

appreciably to the volume of traffic.) 

PTC uses an RF data link to communicate messages to and from trains. A limited 

amount of frequency spectrum is available to the railroads. Mobile RF data links incur 

dynamic signal degradation due to factors such as interference, thermal noise, contention, 

multipath and signal blockage. All of these factors limit the amount of data link 

throughput achievable as well as the message reliability and timeliness. 

The primary ramification of insufficient communications system performance is a 

reduction in train throughput on the line, for the following reasons. Because of the 

communications-based architecture of PTC systems, message delays and limitations in 

31 
train control data link throughput and message reliability can limit train throughput. The 

objective is for a train to receive its next movement authority update before it gets within 

braking distance plus warning time (distance) of the limit of its current authority. If the 

data link delivers a movement authority update too late, the associated train may have to 

reduce speed. If the data link does not deliver a movement authority update at all, the 

associated train will have to stop when it reaches the end of its last movement authority. 

If the data link fails to deliver train location updates, the server cannot advance the 

movement authority for a following train. 

The greater the density and speed of trains under the coverage of any one data radio base 

station, the greater the data link throughput and message reliability requirements. The 

lower the message reliability, the greater the average communication delay, as the system 

must wait for message retransmissions. Also, the greater the update rate of location 

reports and movement authorities, the shorter the headway between trains (the closer they 

can get to the theoretical minimum spacing, which approaches the following train’s 

braking distance). In addition, RF link loading is significantly increased by heartbeat 

messages. Heartbeats are messages sent frequently from all safety-critical nodes to 

indicate that they are still alive and can still be heard. Scenario-specific statistical 

analysis is required to quantify the above relationship between communication-system 

performances and PTC system performance. 

4.3.2 Potential Solutions 

The following are methods to increase the usable capacity and message reliability of an 

RF train-control data link. 

• Increase data rate: This generally requires more bandwidth or higher order 

modulation. Frequency spectrum is precious, so more bandwidth may not be 

available. Practical limits exist on the order of modulation in mobile 

environments. The higher the order of modulation, generally the more 

sensitive is the data link to interference and distortion. Power amplifiers must 

be more linear (expensive). Also, channel equalization may be required. 

These factors can make the radios significantly more expensive. The 

increased sensitivity to noise means that co-channel transmitters will more 

severely reduce usable channel capacity in single frequency systems and more 

channels are required in frequency re-use systems (greater distance is required 

between base stations using the same frequency channel). 

• Optimize protocol parameters: The performance of any communications 

protocol is highly dependent upon key protocol parameters, such as 

retransmission times and delay from the time a channel is sensed until 

transmission occurs in carrier sense multiple access (CSMA) or busy bit 

protocols. Optimization of these parameters for the specific train control 

application can significantly improve performance over default parameters. 

Selecting continuous transmit mode for busy bit base stations can appreciably 

reduce contention. 

32 
• Apply frequency reuse: Assigning different duplex frequency channel pairs to 

neighbor and next neighbor base stations, for example, allows continuous, 

simultaneous transmission and reception at all sites. The higher the required 

signal to noise ratio of the chosen modulation (as with higher order 

modulations), the farther away must be the nearest co-channel neighbor. 

When frequency re-use is used, the mobile units must have an algorithm for 

changing to the correct channel when handing off from one base station to the 

next. 

• Obtain more channels: Often, high capacity is required at only a small 

percentage of locations. It may be possible to acquire more frequency 

channels in those areas without requiring the additional channels elsewhere. 

• Eliminate link access contention: The RF data link in bound to a base station 

typically involves multiple trains contending for the same channel. 

Contention-based link access protocols (LAP) do not permit efficient use of 

available channel capacity. For example, the Aloha protocol has a theoretical 

maximum of 17 percent channel throughput (considerably less in most 

practical applications) and Slotted Aloha has a theoretical maximum channel 

throughput of 35 percent. CSMA and busy bit (CSMA’s duplex equivalent) 

LAPs can perform better, but factors such as hidden terminals can limit 

CSMA performance on RF links. Noncontinuous base station transmission is 

typically used and can limit Busy Bit performance. Elimination of contention 

can significantly increase efficiency of channel utilization. Polling is a simple 

method of eliminating contention. However, polling wastes bandwidth 

because of guard times required when there is uncertainty in the 

communication delays, which is often the case with mobile users. Time 

division multiple access (TDMA) can permit nearly 100-percent channel 

utilization, but it is more complex to manage and the degree of efficiency is 

dependent upon the predictability of the message traffic patterns. TDMA 

often requires synchronization of application and communication frames to 

achieve optimum performance. Commercial off-the-shelf (COTS) or COTSderived solutions are generally preferred for train-control applications since 

development of a new data link and/or protocol can be very expensive and 

time consuming. COTS solutions also offer the opportunity to benefit from 

economies of scale. However, COTS solutions do not generally accommodate 

application-specific features, such as frame synchronization. 

33 
4.3.3 NAJPTC Approach and Rationale 

The U.S. railroad industry has six ultra-high frequency pairs with 12.5 kHz channel 

spacing assigned for train-control use. The protocol standard for use on these train 

control channels is of 1980’s vintage and is known as Advanced Train Control System 

(ATCS) Specification 200 (Spec-200, now known as AAR Std. S-5800). The Spec-200 

standard has a user data rate of 4,800 bits per second with a busy-bit contention protocol 

(duplex equivalent of CSMA). The ATCS protocol includes several priority levels, 

which are very important to avoid delaying time and safety critical messages in queues 

behind less critical messages. It also provides different types of service including 

datagram and reliable message delivery. 

Initial analysis and requirements indicated that the current ATCS standard operating on a 

single channel pair in noncontinuous base station transmit mode could barely meet the 

needs of the PTC corridor in Illinois because of the low density of train traffic on that 

line. Because of this and for interoperability reasons, Spec-200 was the type of RF data 

link selected for the NAJPTC project. However, evolving requirements made the ATCS200 data radio system’s throughput insufficient for NAJPTC’s needs. This included the 

requirement (identified later in the project) that the system fail safe within 20 s after a 

critical wayside device ahead of the train failed or could no longer be heard from. 

Meeting this requirement required a high rate of heartbeat messages that well exceeded 

the capacity of Spec-200 radios. 

With the higher rate of heartbeat messages to accommodate a 20-second fail-safe 

response time, the following peak levels of RF data link loading were computed for 

NAJPTC scenarios. The peak demand data was obtained from the FRA-sponsored highperformance data radio project. 

Scenario: Peak Demand (approximate): 

1. High Density Crossings 2,400 bps 

2. High Speed Moving Block 6,400 bps 

3. Multiple Meet/Pass 6,400 bps 

4. Triple-Track High Density 15,864 bps 

These peak-demand levels (which include Spec-200 upper-layer protocol overhead) 

significantly exceed the throughput capacity of Spec-200 data link inbound channel, 

which is approximately 1/10 of the 4,800 bps channel rate (480 bps) when operated at the 

required message first try success rate of 0.9 to 0.95 when contention, bit errors, 

multipath, and overhead are taken into consideration. The triple-track, high-density 

scenario could not occur on the NAJPTC line, but is shown here for purposes of 

extrapolation to a worst-case scenario in other potential territories. 

Route integrity data (predominantly heartbeats from wayside devices) accounts for 38-58 

percent of the demand in Scenarios 1-3 and 71 percent of the demand in Scenario 4. This 

data may not be required in some nonvital PTC implementations. Advance activation of 

highway crossings (not included in some PTC systems) accounts for 7–20 percent of the 

demand in Scenarios 1–3. It was not included in Scenario 4. 

34 
Possible solutions considered specifically for the NAJPTC line were as follows: 

• Continuous transmit mode (achievable with existing Spec-200 equipment), 

• Frequency reuse (achievable with existing Spec-200 equipment), 

• Additional channels (may require modifying ATCS RF synthesizer), 

• Change to a noncontention protocol (polling or TDMA) but maintain other 

ATCS characteristics, and 

• Increase data rate by changing to quadrature phase shift keying or other 

modulation of similar order and efficiency while maintaining other ATCS 

protocol characteristics. 

It was concluded that a higher performance data radio is needed. A project has been 

initiated under alternate funding to specify the requirements to develop and test a new 

generation interoperable data radio system suitable for PTC. 

35 
5.0 Lessons Learned 

Besides the specific technical issue and lessons discussed in the previous section, the 

following are descriptions of other lessons learned from the NAJPTC project. 

• Communications requirements: It is very important for the communications 

system design to take into account the communications demand (load) as a 

function of time and the impacts of latency and lost messages upon overall 

PTC system performance. 

The train throughput capacity of a PTC system is proportional to its data radio 

throughput capacity, which is determined by its link access protocol, channel 

rate, and other characteristics of the inbound radio link. The system design 

cannot be predicated upon a failure-free, first-time message success rate. 

For example, when a location report message does not get through the first 

time, it must be resent after some delay. This causes a variable message 

latency that will force a following train to operate further behind a preceding 

train than is achievable in an ideal implementation. This will be highly 

dependent on the first-time message success rate—a communication system 

with a 95-percent success rate will permit shorter average headways than a 

system with a 90-percent success rate, all else being equal. 

The overhead required for a closed-loop system associated with heartbeats and 

message acknowledgements required for ensuring that vital messages are not 

missed needs to be considered in the specifications. This is particularly 

important to ensure safety when some form of movement restriction must be 

subsequently applied within or to an already-issued authority. 

Since moderate and long-range mobile radio links (the type required for line 

of road communications) are generally very bandwidth-limited, every effort 

must be made throughout the development phase to minimize the size and rate 

of high priority messages transmitted over the RF data link. 

• Message sequencing: Even when the system is operating properly, because of 

the nature of data radio communications, some messages may be lost and need 

to be retransmitted. Consequently, messages will sometimes be received in a 

different order than that in which they were generated. 

As a critical example, an NAJPTC system requirement was to withdraw an 

authority when a controlled signal goes to “stop” if the train has not already 

reached that point. But if the message reporting the signal change reaches the 

office segment before the train location report, the authority may be 

withdrawn in error. To solve this issue (which is common to all PTC systems 

operating in track circuited territory and was referred to as the “train-pastsignal” issue), it is recommended to have the decision—as to whether or not a 

train caused the nearby signal to go red—be made on board by that train, 

because it alone has continuously available, current train location data. 

36 
A related problem experienced on the NAJPTC project was that of correlating 

location information from a variety of sources, especially when discrepancies 

were among the reports or communication delays were incurred on reports 

from some sources but not from others. Communicating trains, for example, 

reported their location to the server as determined by their onboard LDS units. 

In addition, the server received location-related information on the same trains 

from “on-sheet” (OS) circuits (which reported quickly, only incurring 

communication delays) and from intermediate track circuits (which had 

greater delay variations due to coded track circuits in addition to 

communication delays). Furthermore, both ends of each intermediate track 

circuit reported on the same intermediate track circuit independently of one 

another. 

The numerous sources of location-related information (and delays in reporting 

that information) leave system designers with the dilemma of having to 

choose whether the system should immediately assume a train has moved to a 

new location on the basis of a single report from whichever sensor reports 

first, or wait until reports have been received from all relevant sensors, or 

something in between. Requiring the most conservative response (from a 

safety perspective) is not necessarily the best solution in cases where it only 

achieves safety overkill at the expense of excessive train delays. 

Certain PTC objectives or functions are best satisfied by one assumption, 

whereas others favor a different assumption. The issue of message delivery 

with latency that varies must be considered in the design and provisions need 

to be made to accommodate messages from different sources being delivered 

with unpredictable sequences. 

• Braking algorithms: An issue with the braking algorithms used was the 

conservatism required in the enforcement algorithms. NAJPTC was designed 

to feature positive enforcement of authority and speed limits to provide for 

added safety. 

Predictive braking accuracy is particularly important when a railroad wants to 

increase the capacity of a route using moving-block train control in which 

closer spacing of trains is the objective. The problem with trying to calculate 

the braking distance for a full-service application for enforcement purposes is 

that many braking variables are simply not known or only known to an 

approximation. These include brake valve type, brake-rigging efficiency, 

brake shoe composition, piston travel, and actual train weight. An additional 

problem is the inability to predict whether an engineer will use dynamic 

braking and whether or not the independent (locomotive) brake will be bailed. 

This uncertainty leads to further conservatism in braking algorithms. 

Braking algorithms are required to be robust enough that only a miniscule 

probability occurs for allowing a train to get past an authority limit, and 

therefore they must take into account the probability of bad braking 

37 
characteristics. This is done by adding margin to the braking algorithm, 

which must also account for front and rear of train location uncertainties. 

The warning associated with a conservative braking algorithm may cause 

locomotive engineers to operate their trains more conservatively than they 

otherwise would, based on their knowledge of train’s braking characteristics 

(see the prior discussion on consist determination and verification for more 

details and potential solutions). A thorough statistical analysis should be 

performed to ensure that the probability of a train getting past its authority 

limit is not specified with excessive conservatism. This can have drastic 

effects on system feasibility. 

• COTS safety case: Developing a safety case for the use of COTS hardware 

and software in a vital system was very challenging and not completely solved 

on the NAJPTC project. Most importantly, the system developer and safety 

assessors had limited insight into the internal design of COTS components to 

allow safety verification and validation. In addition, the project dealt with 

differences in interpretation of the FRA’s new rule (i.e., the FRA’s “Standards 

for Development and Use of Processor Based Signal and Train Control 

Systems,” 49 CFR 236 Subpart H issued in 2005), because NAJPTC was the 

first vital train control system to fall under its regulation. 

Aside from these challenges, COTS promises a number of potential benefits in 

the design of a vital train-control system such as NAJPTC. These include 

reduced cost and schedule for system development, improved reliability and 

maintainability, state-of-the-art components and development tools, 

availability, and ease of incorporating product improvements. It is worth 

noting, however, that not all of these benefits may necessarily materialize for 

vital train-control applications because of the extra measures and mitigations 

required to attain the very high level of safety integrity required. 

• Vital tracking of noncommunicating trains: The particular architecture 

selected for the NAJPTC project required vital tracking of noncommunicating 

trains, via track circuits and assignment of PTC authorities to 

noncommunicating trains (as well as communicating trains). This 

requirement was not fully solved, and it is highly recommended that an 

alternative architecture be developed that does not require vital tracking of 

noncommunicating trains via track circuits. Or, perhaps a solution might be 

devised that can tolerate more coarse tracking of noncommunicating trains. 

A possible alternative solution/migration path for a line equipped with CTC 

would be to progressively equip trains with PTC and have it operate 

transparently in the background without the ability for displaying authorities. 

Trains would operate according to wayside signals and regular bulletins. 

Authority limits and bulletin restrictions would be transmitted to the train for 

enforcement purposes, and trains would be provided with a locomotive 

display for the purpose of entering required data and for providing warnings 

38 
of speed violations. 

Under this migration path, communicating trains would be fully protected 

against other communicating trains. Partial protection against 

noncommunicating trains might be gained by applying a railroad-configurable 

speed restriction on communicating trains approaching a point of potential 

convergence with a noncommunicating train. Safety would be enhanced as 

more and more locomotives became equipped. When enough locomotives 

became equipped to warrant the removal of the signal system (or intermediate 

signals), the system could then permit the display of authorities and 

restrictions. This transition could be facilitated through functional negotiation 

or through upgrading of the onboard system. 

The key advantage of this migration path is that it would avoid the costly step 

of installing PTC equipment (e.g., wayside interface units (WIU)) at 

intermediate signal/track circuit locations only to have them eventually 

discarded upon transition to standalone PTC mode. 

• Train control design philosophy when using two fail-safe systems: One of the 

problems encountered in the NAJPTC project occurred when one of two failsafe devices that measured the same condition (track occupancy) failed. The 

design philosophy was to assume the worst-case condition; namely, that the 

track was occupied, even though the other fail-safe device was measuring no 

occupancy. This led to incorrect conclusions about the location of unequipped 

trains on more than an acceptable number of occasions. 

A better design philosophy would have been to accept the less restrictive 

condition of the two fail-safe devices measuring the same condition. This will 

reflect the actual condition more accurately than using the worst-case 

condition. Because both devices were fail-safe, no unsafe condition should 

result from this less restrictive approach. In the case of the NAJPTC project, 

this approach would have avoided incorrect conclusions about the location of 

trains as only real occupancies would have been reported as occupancies. 

• Incremental development: The NAJPTC project was one of the most complex 

system development/integration projects undertaken to date. The project was 

to develop a complex, highly distributed system that included mobile nodes 

and was required to meet stringent safety, availability, and performance 

requirements. The requirements for NAJPTC far exceeded those of any traincontrol system previously implemented. 

The original plan called for seven builds, which was probably the minimum 

number of builds feasible for a project of this magnitude. Unfortunately, the 

required schedule for project completion was too short to accommodate seven 

builds, so the number of builds was reduced, but the total functionality was 

not. This resulted in the complexity of the second build becoming so high that 

it proved to be unachievable in a single build. 

39 
So, the greatest lesson illustrated by the NAJPTC project was that such large 

systems must be developed incrementally. This relates to the lessons 

discussed later regarding visible and quantifiable progress, sequence of 

development, and no short-cuts. 

• Thorough specifications: To develop PTC systems requires much more 

railroad operational knowledge than required for the signal systems they 

replace, knowledge that not every system developer may have. Therefore, 

procurement specifications must be extremely thorough, unambiguous, 

understandable at all levels, and correct. System engineers well trained in 

effective specification writing and experienced with railroad operations and 

control system technologies are necessary to generate such specifications. 

It is also useful to prepare a concept of operations that mirrors the 

specifications in language appropriate for a nonexpert to provide insight as to 

why the system specification says, “The system shall do this. . . . .” 

• Knowledge transfer: It is necessary for the system developer to understand 

the rationale behind the specifications; i.e., why each requirement is there and 

the associated underlying train operations. This should begin with a thorough 

meeting soon after (or preferably before) the system development contract is 

signed so that domain knowledge is transferred early. 

If the design engineers are not familiar with railway operations, they need to 

spend time on the railway with operating officers to gain domain knowledge. 

• Teamwork: A cooperative working relationship between the customer and the 

system developer is essential to obtaining a good product on time and within 

budget. An adversarial relationship stifles communication and knowledge 

transfer, which increases cost, extends project implementation time, and 

undermines confidence in the system. 

• Meetings: Although telephone conference calls and e-mails are useful for 

routine communications, face-to-face meetings can be better for problem 

solving and for exchange of critical information. When face-to-face meetings 

are not practical, use of an Internet document review tool is recommended so 

all parties can review and discuss the same document page simultaneously. 

Most important, however, is that all decisions, agreements, and conclusions be 

clearly documented in detail and preserved for future reference. E-mail can 

be beneficial in this regard. 

• Early test planning: The test plan should be developed for use by designers 

early in the project, before the design is finalized, to assure that the system 

design is comprehensive. This test plan may take the form of a requirements 

verification conditions and criteria matrix (RVCCM). 

40 
The RVCCM includes a list of each system requirement to be tested, the 

operational conditions under which it will be exercised and its acceptance 

criteria. The RVCCM also indicates whether each test is to be performed in 

the lab and/or field. The RVCCM provides a different perspective from the 

system specification that is invaluable in correctly interpreting each 

requirement. 

• Sequence of development: In large programs with tight implementation 

schedules, it is imperative that the normal development sequence is not shortcut. Coding should not start before the implications of the requirements are 

fully understood, and before the design is solidified and agreed to. 

Coding before significant design issues are resolved is a recipe for design 

rework later with resulting cost and schedule overruns. 

• No short-cuts: Developing a new PTC system is a large undertaking, 

particularly if it is to be a standalone (vital) system. There are numerous 

facets and variations to typical train moves, each one of which must be 

accommodated by a PTC system. 

Customers must have patience and should be cautious regarding any proposed 

quick, inexpensive development solution that does not specifically address 

details and difficult technical performance and safety issues, such as those 

discussed. 

Developers must have PTC development experience to understand the 

complexities involved and how to adequately address them. 

• Test environment: Even with careful planning and coordination to permit 

system testing on an active rail line, NAJPTC field testing proved to be very 

time consuming and expensive. Test train movements could not interfere with 

revenue trains. This often resulted in the test train and all associated test 

personnel waiting for a window in which to operate between revenue trains. 

Changes in test plans resulted from encountering unexpected events in the 

field. Railroads and regulatory agencies required that extensive 

documentation be generated upon any change in test plans. 

Coordinating among the schedules of numerous participants from the railroads 

involved, FRA, state agencies, suppliers, the NAJPTC project’s Program 

Office (i.e., TTCI), and its system engineer The NAJPTC project’s system 

engineer was extremely difficult and left virtually no flexibility to 

accommodate unexpected events. 

Test operation at high speeds required significant additional precautions, such 

as flagging personnel at highway-grade crossings and absolute block 

41 
protection around the test train. 

Compounding the issue of testing in a revenue track environment was the 

approximate one year lead time it took to implement any change to the WIU 

software (which involved making the change, verifying it in the lab, in the 

field, and by an independent auditor, rolling it out to all sites in the affected 

territory, and then field-verifying it). This was because the WIUs were part of 

the vital revenue signaling system on the line. 

These considerations added significantly to the time and expense incurred 

with field testing. A controlled PTC test bed on a dedicated test track (absent 

of revenue trains and standard railroad regulatory requirements) could have 

benefited the NAJPTC project by avoiding or reducing the above issues. 

• Shadow mode testing: Shadow mode testing placed the NAJPTC system in 

daily operation on select revenue-service locomotives without a brake 

interface and without any driver display or input. The system operated in the 

background with logging occurring in the office and in the onboard computer. 

This permitted considerably more scenarios to be exercised at substantially 

less cost than was possible in manned field testing. 

Shadow mode testing was geared towards wringing out problems associated 

with the vital tracking of noncommunicating trains, integrating the tracking of 

fitted trains using LDS and track circuit information, and the association of 

authorities to trains. 

Shadow mode testing provided a tremendous boost in the amount of 

information available to use in solving problems, especially those that only 

appeared outside the lab. Unfortunately, it was not available until late in the 

project. 

• Visible and quantifiable progress: A key to ensuring the successful 

conclusion of a complex system is to show visible progress throughout a 

relatively long development process. This not only provides a tangible means 

by which to measure progress, but also allows users to gain experience with 

parts of the system and to provide feedback into subsequent development 

stages. This implies implementing, testing, and demonstrating useful interim 

releases of the system that provide ever-increasing levels of functionality. 

A quantitative means of monitoring and demonstrating progress that proved 

very useful was the implementation of metrics or technical performance 

measures (TPM). TPMs that proved to be most useful on the NAJPTC 

project, particularly during shadow mode testing, were as follows: 

− PTC officer server (POS) failures per 24-hour POS operating hours for all 

trains (plotted by week and as a rolling average) 

42 
− False nonexplicit modes per 24-hour POS operating hours for all trains 

(plotted by week and as a rolling average); nonexplicit is a degraded 

operating mode that the POS has entered during which no new or extended 

authorities will be issued because it has detected a discrepancy between its 

status and that of CAD. 

− False nonexplicit modes per 24-hour POS operating hours for 

communicating trains only (plotted by week and as a rolling average) 

− False nonexplicit modes per 24-hour POS operating hours for noncommunicating trains only (plotted by week and as a rolling average) 

− False reports of authority violations per 100 train miles for all trains 

(plotted by week and as a rolling average) 

− False reports of authority violations per 100 train miles for communicating 

trains only (plotted by week and as a rolling average) 

− False reports of authority violations per 100 train miles for 

noncommunicating trains only (plotted by week and as a rolling average) 

− False reports of enforcements per 100 train miles for communicating trains 

(plotted by week and as a rolling average) 

The TPMs were updated and reviewed among the project team weekly and 

presented to stakeholders monthly (see Figures 14 and 15). This provided an 

objective and quantitative means of measuring progress in solving system 

performance problems. 

Weekly Failure Rate - All Trains

0.0

0.5

1.0

1.5

2.0

2.5

30-Jan

6-Feb

13-Feb

17-Feb

24-Feb

3-Mar

10-Mar

17-Mar

24-Mar

31-Mar

7-Apr

14-Apr

21-Apr

28-Apr

5-May

12-May

Rate of Occurrences

POS Failure False Non-Explicit

Figure 14. Example of TPM for POS Failures and Nonexplicit Mode Events 

43 
Weekly Failure Rate - All Trains

0.0

0.1

0.1

0.2

0.2

0.3

0.3

0.4

0.4

0.5

0.5

30-Jan

6-Feb

13-Feb

17-Feb

24-Feb

3-Mar

10-Mar

17-Mar

24-Mar

31-Mar

7-Apr

14-Apr

21-Apr

28-Apr

5-May

12-May

Rate of Occurrences

False Violation

Figure 15. Example TPM for False Reports of Authority Violation 

• System Performance: In developing software-intensive systems, often the 

tendency is to focus only on functionality and interfaces, largely ignoring the 

performance aspects. 

In the development of real-time control systems, performance issues can be 

complex and must not be neglected or postponed, especially for those systems 

having RF communications in the loop. Of particular importance is the 

relationship between RF communications system bandwidth, message latency, 

message reliability, system-response time to changes in safety conditions, and 

train throughput capacity on the PTC corridor. 

Designing for safety and interoperability can adversely affect system 

performance, if performance is not concurrently factored into the design. A 

PTC system with poor performance (e.g., excessive false enforcements or 

increased headways) will not be acceptable. 

To understand complex technical performance issues and to develop an 

efficient, coordinated/balanced total-system solution, it is strongly advisable 

for the development team to heavily involve at least one system engineer with 

expertise in all of the key areas (RF communications, network protocols, realtime control systems, safety, and train control) throughout the project. 

The bottom line is that the design team must address functionality, 

performance, and safety concurrently and integrally. 

44 
6.0 Conclusions 

PTC offers the promise of significant potential benefits in railroad safety, capacity, and 

efficiency. However, PTC reveals a different and much more complex but not 

insurmountable set of design issues than those encountered with traditional train-control 

approaches. Like many railroading problems, the issues associated with train control are 

very scenario-dependent and the solutions must be engineered accordingly. Much more 

so than traditional signaling design, PTC development requires a solid understanding of 

railroad operating rules, because it embodies many of them. Finding the optimum 

solutions to these challenges often requires well-coordinated customer and developer 

trade analyses of potential cost and operational ramifications. 

This report has identified key issues encountered on the NAJPTC project along with 

potential and chosen solutions, as well as the rationale for those choices. The experiences 

gained from this project have benefited other PTC developments and have led to the 

inception of subsequent projects to further address issues identified on this project. 

45 
46 
Appendix A. 

NAJPTC Onboard Equipment 

Figure A-1 shows the installation of the NAJPTC display in a freight locomotive. 

Figure A-1. NAJPTC Display Installed in a P42 Passenger Locomotive 

Figure A-2 illustrates the following primary elements of the NAJPTC display. 

1. Mode indicator: Continuously visible indicator providing maintenance and 

operational mode indication (e.g., power up, initializing PTC, active PTC) 

2. Speed and location: Continuously visible indicator providing textual speed and 

milepost location data. 

3. Speed limit graphic: Represents the most restrictive speed limit for a prescribed 

area in the display horizon. A text box reinforces the speed limit and is located 

within the graphic. The text box will also be used to indicate whether the area is a 

restrictive speed restriction and will contain the text “restrictive” as required. 

4. Vertical speed scale: The speed scale is a linear scale that provides a constantspeed value scale which will adjust vertically in conjunction with the train speed 

pointer to encompass the full range of possible train speeds within a limited 

47 
vertical window area. The speed scale also provides a reference for speed limit 

restrictions in the display horizon. 

5. Train speed pointer: The train speed pointer provides a graphical representation 

of the train’s speed using the speed scale as well as a change in vertical height as 

the train accelerates and decelerates. The vertical distance between the speed 

pointer and upcoming speed-limit graphics provides a comparative relationship 

between the train’s speed and the upcoming speed limits. The train pointer 

remains stationary in the horizontal location and extends down into the bottom 

auxiliary area and into the train position and length indication. 

6. Speed prediction indicator: A graphical vector with two filled-in circles 

representing the train’s predicted speed and location in 30 s and 60 s, respectively. 

The upward and downward direction of the vector represents if the train is 

predicted to accelerate or decelerate, respectively. 

7. Speed change or authority limit bar: This vertical bar represents a change in 

speed or end of a train’s authority to move (speed = 0). The bottom of this bar 

contains a graphical hint of the lower speed limit if it is currently off the scale 

with respect to the train’s current speed. 

8. Train location scale: The train location scale is a linear scale that provides an 

absolute reverence of the location of graphical entities (track profile data, speed 

limit boundaries, speed prediction indication, and work limit) with respect to the 

front (point movement) of the train. The scale is fixed and has major scale marks 

in miles and minor scale marks of 2/10ths of a mile. The scale extends at least 6 

mi in the front of forward motion of the train and at least 1/4 mi from the rear of 

forward motion of the train. 

9. Train Position and Length Indicator: This indicator is connected to the train 

speed pointer and varies in horizontal length, which represents the length of the 

train. The length measurement is relative to the train location scale. The position 

of the train with respect to track profile icons can be determined from the train 

location scale. 

10. Track profile indicators: Track profile indicators consist of mileposts, crossings, 

control points, hot box, and defect detectors. Track profile indicators move along 

the horizontal and relative to the speed of the train. 

48 
Figure A-2. Primary Elements of the NAJPTC Display 

Figures A-3, A-4, and A-5 show remaining NAJPTC onboard hardware items installed on 

a locomotive. 

Figure A-3. Locomotive Interface Module, Location Determination System, 

Onboard Computers 

49 
Figure A-4. Onboard Mobile Communications Package 

Figure A-5. Onboard Antennas 

50 
NAJPTC Wayside Equipment 

Figures A-6, A-7, A-8, and A-9 show a typical WIC installation. The primary 

components are a Microlok electronics cabinet, an ATCS Specification-200 Mobile 

Communications Package (MCP), and antennas for the MCP and global positioning 

sensor. 

Figure A-6. Typical WIU Housing 

Figure A-7. Typical Microlok and MCP Installation in WIU Housing 

51 
Figure A-8. Typical GPS Antenna Installation on WIU Housing 

Figure A-9. MCP Antenna Outside WIU Housing 

52 
NAJPTC Office Equipment 

Key office equipment elements are the quad-redundant POS shown in Figure A-10 and a 

remote terminal shown in Figure A-11. 

Figure A-10. NAJPTC POS 

Figure A-11. POS Remote Terminal 

53 
54 
Appendix B. 

Status of NAJPTC Implementation by Function 

The NAJPTC project was terminated during the Build 2 phase. At the time of project 

termination, all Build 1 functionality and much of the Build 2 functionality had been 

implemented and successfully tested. Table B-1 provides the status of each function at 

the time of project termination. The status of each function is defined in the following 

terms: 

w working (tested & demonstrated) 

pw partially working 

nw not working 

nt not tested. 

Table B-1. NAJPTC Build 1 and 2 Functions S 

T 

A 

T 

U 

S 

System 

Start-up 

Verify health of all components (Server, Communications, 

Onboard computers (OBC), WIUs) 

w 

Verify correct software version of OBCs WIUs (Software Version 

Control) 

w 

Ongoing 

Monitor health of all components (Health Monitoring & 

Diagnostics) 

w 

Shut down vital component if failure detected w 

Time Synchronization w 

System Response to failed OBCs w 

System Response to failed WIUs pw 

Location Management 

Train Initialization 

Acquisition/Verification of Route Data w 

Acquisition of consist data w 

Acquisition of bulletin data w 

Departure Test w 

Onboard Location Tracking w 

55 
Track Resolution 

Semi-automated at start-up w 

Automatic standalone tracking through switches w 

Onboard Display of Train Location w 

Onboard Display of Train Speed w 

Onboard Display of Track Profile and Characteristics w 

Onboard Display of Route w 

Server Location Tracking of Trains and Vehicles 

Communicating Trains pw 

Noncommunicating trains by track circuit nw 

Reporting train location to CAD 

Communicating pw 

Noncommunicating pw 

Remote train location reporting 

Communicating nt 

Train Termination of PTC functions w 

Authority Management 

Authority Issuance 

Signal-based pw 

Forms-based (with written instructions) pw 

Display Authority and Instructions - Text Format w 

Display Authority and Instructions - Graphic Format w 

Automatic Release of Authorities w 

Authority Cancellation (by train dispatcher) w 

Authority Modifications w 

Authority Verification 

Safety Check - verify safe to issue pw 

Seek and verify acknowledgement from joint holders, if applicable nt 

Ensure that authorities are not delivered to train before applicable 

bulletins 

w 

Incremental Authorities - Moving Block for Communicating trains pw 

Broken Rail Detection (unidentified track occupancy) w 

Speed Management 

Deliver Civil Speed Limits to trains as required w 

Temporary Speed Restrictions (TSR) (Form A or V) 

Receive TSRs from CAD or other source w 

Distribute TSRs to affected trains on initialization or when 

received 

w 

Maintenance of Way Protections (MWPs) (Form B or W) 

Receive MWPs from CAD or other source w 

Distribute MWPs to affected trains on initialization or when 

received 

w 

Advisories 

Receive Advisories from CAD or other source w 

56 
Distribute Advisories to affected trains on initialization or when 

received 

w 

 Apply Lading/Equipment Speeds Restrictions as required w 

 Apply Speed Restrictions based on Route Integrity Monitoring 

 Restricted Speed (head end only) for track circuit anomaly, slide 

detected, etc. 

w 

Stop & Inspect for potential open switch nt 

Apply restriction at nonfunctional HRX or for extended activation nt 

Onboard Display of Integrated Speed Limits and Restrictions w 

Train Integrity Monitoring 

Train Intact/Cut Status nt 

Detect Emergency Brake Application nt 

Route Integrity Monitoring 

Monitor Track Occupancy by Signal Block pw 

Monitor Signals pw 

Monitor Power-Operated Switch Position and Control Point 

Occupancy 

pw 

Monitor Hand-Operated Switches pw 

Crossing Monitoring and Activation 

 Activate Highway Rail Crossing Warning Devices nt 

Monitor Health of HRI System nt 

Emergency Warnings 

Emergency Brake Application Warnings nt 

Authority Violation Warnings nt 

Unauthorized Movement Warnings w 

Response to Emergency Warnings 

Emergency Reduction of Authority or Application of Restricted Speed nt 

Warnings and Enforcement 

Identify Enforcement Targets 

Entry point to PTC w 

Authority Limits w 

S&I Restrictions w 

MWP Limits w 

Speed Limits and Restrictions w 

Predictive Enforcement Warnings 

Entry point to PTC w 

Authority Limits w 

S&I Restrictions w 

MWP Limits w 

Speed Limits and Restrictions w 

57 
Predictive Enforcement 

Entry point to PTC w 

Authority Limits w 

S and I Restrictions w 

MWP Limits w 

Speed Limits and Restrictions w 

Reactive Enforcement Warnings 

Speed Limit Ceilings w 

Restrictions imposed over train occupancy nt 

Reactive Enforcement 

Speed Limit Ceilings w 

Restrictions imposed over train occupancy nt 

Enable/Disable Enforcement w 

Detect and Report Enforcement enable/disabled nt 

Notification when enforcement is invoked w 

Functions Required 

Functions tested & demonstrated (w) 55

Functions partially working (pw) 12

Functions not working (nw) 1

Functions not tested (nt) 14

58 
Appendix C. 

Quantification of Potential Increased Capacity and Reduced Headway 

with Vital, Moving Block PTC 

Methodology 

The potential headway reductions and capacity increases that could be gained by using a 

vital, moving block PTC system are analyzed and presented here. Trains were modeled 

operating in a following move scenario at the same speed (e.g., double track current-oftraffic, or fleeting on single track), with the results compared against a conventional 4-

aspect signaling system. Because PTC Standalone mode was a Build 2B feature and 

Build 2B was not funded, the potential benefits of moving block have not been proven by 

actual implementation and field testing. 

The simulation parameters and results are for the case of a 123-car loaded grain train 

powered by four SD-40 locomotives, traveling at prescribed speeds, considering various 

grades, block lengths, warning times, and brake algorithm margins. 

The average train stopping distances (using a nominal full-service braking application) 

were obtained from TOESTM simulations, whereas the rest of the parameters are from 

NAJPTC requirements, design capabilities, or typical railroad signaling system attributes. 

Every attempt has been made to make the predictions of PTC performance accurate by 

using actual NAJPTC system design parameters, braking algorithm margins, and 

nonidealities. Figure C-1 shows how these parameters relate to the scenarios. 

Direction of Movement

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation 4-Aspect

Signaling

Fixed

Block

Moving

Block

Speed

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

•Train Integrity Loss Detection

Safety Buffer

Braking Curve

•Signal Sighting Time* •Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Direction of Movement

4-Aspect

Signaling

Fixed

Block

Moving

Block

Speed

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

•Train Integrity Loss Detection

Safety Buffer

Braking Curve

•Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

•Signal Sighting Time*

Figure C-1. Relationship of Moving Block and Conventional Signaling Parameters 

Used in Analysis of Potential Headway Reduction and Capacity Increase 

59 
Unless otherwise noted, the results presented are for a pure moving block PTC system. 

As such, the models assume the use of a vital train integrity detection device (i.e., that 

track circuits are not required for train integrity protection). The NAJPTC Standalone 

Territory mode implementation also accommodated a hybrid moving block/fixed block 

mode of operation. The hybrid mode would use track circuits for train integrity 

protection, resulting in a speed restriction being enforced through an occupied block 

ahead. Results for the hybrid mode of operation are presented as well. 

The characteristics of the conventional case modeled are: 

• Both leading and following trains are operating at the same, constant, specified 

speed 

• 4-aspect signaling with coded track circuits (signal tumble-down time 

considered). Aspects defined as: 

− Restricting—proceed at restricted speed. 

− Approach—proceed prepared to stop before any part of train or engine passes 

the next signal. Freight trains exceeding 30 mph must immediately reduce to 

30 mph. Passenger trains exceeding 45 mph must immediately reduce to 45 

mph. 

− Advance approach—proceed prepared to stop at second signal. Trains 

exceeding 40 mph must immediately reduce to 40 mph. 

− Clear—proceed. 

• The 4-aspect signaling results in the following minimum train separations: 

− 3 blocks separation for train speeds above 40 mph; 

− 2 blocks separation for train speeds of 31–40 mph; 

− 1 block separation for train speeds of 21–30 mph; 

− Full-service braking distance separation for trains in same block with speeds 

≤ 20 mph. 

Analysis results are provided for trains traveling at 60, 40, and 30 mph. 

Table C-1 shows the simulation parameters and results for the loaded grain train traveling 

60 mph on a level route, using a block length of 2.5 miles, a 20-second warning time, and 

the NAJPTC brake algorithm margin. 

60 
Table C-1. Analysis of Potential Headway Reduction and Capacity Increase for 

Moving Block PTC vs. 4-Aspect Signaling Using 2.5-Mile Block Length for a Train 

Traveling 60 mph on a Level Route 

Train Simulated: 123-car loaded grain train with 4 SD-40 locomotives 

Parameters Used in Analysis: 

Route 

0% grade

Train 

Speed: 60 mph 

Length: 6,798 ft 

Weight: 14,088 tons 

Moving Block and 4-Aspect Fixed Block Signaling Parameters 

feet 

3 block lengths 39,600 (2.5 miles/block) 

 + signal tumble-down 1,584 (6 sec/block) 

 + signal sighting time 704 (8 sec) 

 - average train stopping distance 7,531 (7,531 ft per TOES, using fullservice braking) 

 - brake algorithm margin 1,762 (1,762 ft per NAJPTC braking 

algorithm) 

 - LR-MA latency1 1,320 (15 sec/LR) 

 - location uncertainty (10 ft per NAJPTC System 

Specification requirement) 10

 - warning time (to crew) 1,760 (20 sec) 

 - train integrity loss detection 1,760 (20 sec) 

Potential Headway Reduction 27,745 ft (or 315 seconds) 

Potential Capacity Increase 133% (factor of 2.33) 

1

LR-MA latency refers to the inverse of the movement authority update rate, which 

generally equates to the location report rate for the case of close following moves.

61 
Baseline Case, Running over Level Route 

The following results (Figures C-2 through C-4) are for the baseline case in which the 

train is operating over a level route. The full-braking margin is used with a 20-second 

warning time to the engineer before enforcement. It is also assumed that a vital train 

integrity protection device is used, so that no enforced speed restriction is necessary 

through an occupied block ahead. Figure C-1 illustrates the simulation parameters. 

Train Speed = 60 mph 

 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

114

227

341

455

568

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-2. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 60 mph on a Level Route, 

Using a 20-Second Warning 

62 
Train Speed = 40 mph 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 40 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

170

341

511

682

852

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 40 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-3. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 40 mph on a Level Route, 

Using a 20-Second Warning 

63 
Train Speed = 30 mph 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 30 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

227

455

682

909

1136

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 30 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-4. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 30 mph on a Level Route, 

Using a 20-Second Warning 

64 
 Baseline Case, Ascending a 1 Percent Grade 

For trains traveling up a grade, the improvements in potential headway reduction and 

capacity increases become even more apparent. If the same train is analyzed traveling up 

a 1 percent grade at 30 mph, the potential headway reduction and capacity increases 

provided by the vital, moving block PTC system are as shown in Figure C-5. Note that in 

this case, the block lengths have been increased proportionately to account for the 1 

percent grade, assuming the same signal locations are used for trains ascending and 

descending the grade. 

Train Speed = 30 mph 

0

10,000

20,000

30,000

40,000

50,000

1.75 2.1 2.45 2.8 3.15 3.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 30 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 1%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

227

455

682

909

1136

0

0

50

100

150

200

250

300

1.75 2.1 2.45 2.8 3.15 3.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 30 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 1%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-5. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 30 mph up a 1% Grade, 

Using a 20-Second Warning 

65 
Increase Warning Time to Engineers before Penalty Enforcement 

Increasing the enforcement warning time to engineers negatively impacts potential 

headway reduction and capacity increases. Returning to the scenario in which the train is 

traveling 60 mph on a level route and increasing the warning time to engineers from 20 

seconds to 40 seconds, the resulting potential headway reduction and potential capacity 

increases are illustrated in Figure C-6. For a block length of 2.5 miles, the potential 

capacity increase has dropped from 133 percent to 115 percent compared to the case in 

which the warning time is 20 s. 

The negative impacts in potential headway reduction and potential capacity increases 

(compared to the scenario with a 20-second warning) are due to the fact that the earlier 

engineers receive the warning that penalty braking enforcement is imminent, the more 

likely they are to brake the train sooner than is necessary to stop the train in time. 

Warning Time = 40 Seconds 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 40 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

114

227

341

455

568

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 40 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-6. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 60 mph on a Level Route, 

Using a 40-Second Warning 

66 
Reduce Braking Margin 

One area in which PTC technology needs further research and development is in braking 

algorithms. The variability of performance within each train’s braking system requires a 

braking margin or safety net to be added in to the predicted braking distance. If the PTC 

system would know the train’s braking characteristics with greater accuracy, this margin 

could be reduced. For example, if the representative model’s braking margin would be 

cut in half (from 1,762 ft to 881 ft), then the potential headway reductions and potential 

increases in capacity would improve as Figure C-7 shows. In this case, capacity could 

potentially increase by 10 percent when compared to the case in which the full-braking 

margin is used. 

The theoretical performance limit (best case) for a PTC braking algorithm would 

eliminate the braking margin altogether. The potential headway reductions and potential 

increases in capacity would further improve, leading to the results seen in Figure C-8. In 

this scenario, capacity could potentially increase by 21 percent when compared to the 

case in which the full-braking margin is employed. It is not likely that this ideal case will 

ever be achieved. 

An FRA-sponsored project has been initiated to develop a more accurate braking 

algorithm that will adapt by measuring the actual braking performance and related 

characteristics of each specific train. 

67 
Halved Braking Margin 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Half Current

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

114

227

341

455

568

0

 

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Half Current

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-7. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 60 mph on a Level Route, Using a 

20-Second Warning, and Half the Current Braking Margin 

68 
Zero Braking Margin 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Zero

 Warning Time: 20 sec

 Enforced Speed Restriction: No

Potential Headway Reduction (sec)

114

227

341

455

568

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Zero

 Warning Time: 20 sec

 Enforced Speed Restriction: No

4X

3X

2X

 

Figure C-8. Potential Headway Reduction and Capacity Increase for Moving Block 

PTC vs. 4-Aspect Signaling for a Train Traveling 60 mph on a Level Route, Using a 

20-Second Warning, and Zero Braking Margin (Ideal Case) 

69 
Hybrid Moving Block/Fixed Block—Moving Block with Track Circuits Used for 

Train Integrity Protection 

A hybrid moving block/fixed block vital, positive train control system would use track 

circuits for train integrity protection. The train integrity loss detection parameter seen in 

Figure C-1 does not apply, because this approach relies on track circuits and a restricted 

speed restriction in the block occupied by the leading train for protection against pullaparts (i.e., no vital EOT is in this configuration to detect a pull-apart within 20 s). This 

is the mode of operation designed for use in NAJPTC standalone territory. Figure C-9 

shows the simulation parameters used for the hybrid moving block/fixed block vital, 

positive train control system. 

Direction of Movement

4-Aspect

Signaling

Fixed Block

Hybrid 

Moving Block/Fixed Block

Speed

•Signal Tumble-Down Time*

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

Safety Buffer

Braking Curve

•Signal Sighting Time* •Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

Up to 1 block

60 mph

20 mph

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Direction of Movement

4-Aspect

Signaling

Fixed Block

Hybrid 

Moving Block/Fixed Block

Speed

•Signal Tumble-Down Time*

•Brake Algorithm Margin

•Location Report-Movement Authority Latency

•Location Uncertainty

•Warning Time

Safety Buffer

Braking Curve

•Signal Sighting Time* •Signal Tumble-Down Time*

*Times shown above as equivalent distance by multiplying by mph

Up to 1 block

60 mph

20 mph

≥ Braking Distance

of Worst Case Train

at Track Speed (>40 mph)

Excess Train

Separation

Figure C-9. Relationship of Moving Block and Conventional Signaling Parameters 

Used in Analysis of Potential Headway Reduction and Capacity Increase for a 

Hybrid Moving Block/Fixed Block PTC System Using Track Circuits for Train 

Integrity Protection 

The simulations for this scenario (results shown in Figure C-10) assume that the enforced 

speed restriction through the occupied block ahead is equal to 20 mph, the upper limit of 

a restricted speed restriction. Increasing the enforced speed restriction to 30 or 40 mph 

would bring system performance closer to that of true, moving block, but risk analysis 

would be needed to justify raising the restricted speed from 20 mph. The NAJPTC 

system included a railroad configurable parameter for this speed restriction to allow the 

20-mph hybrid mode described above, pure moving block, or variations in between. 

70 
Train Speed = 60 mph, with Enforced Speed Restriction of 20 mph 

0

10,000

20,000

30,000

40,000

50,000

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Headway Reduction (ft.)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec 

 Enforced Speed Restriction: Yes

Potential Headway Reduction (sec)

114

227

341

455

568

0

0

50

100

150

200

250

300

1.25 1.5 1.75 2 2.25 2.5

Block Length (miles)

Potential Capacity Increase (%)

123 car Loaded Grain Train

Motive Power: 4 SD-40s

Speed: 60 MPH

Length: 6,798 ft

Weight: 14,088 tons

Grade: 0%

 Braking Margin: Full

 Warning Time: 20 sec

 Enforced Speed Restriction: Yes

4X

3X

2X

 

Figure C-10. Potential Headway Reduction and Capacity Increase for Hybrid 

Moving Block/Fixed Block PTC vs. 4-Aspect Signaling for a Train Traveling 60 

mph on a Level Route, Using a 20-Second Warning, and an Enforced Speed 

Restriction of 20 mph in the Occupied Block Ahead 

It should be noted that while moving block (“standalone”) PTC has the potential to 

increase railway capacity and reduce headways, the Integrated mode (or an overlay PTC 

system) cannot increase capacity—it can only degrade capacity and headway (due to 

braking algorithm margin and system delays), because it imposes additional constraints 

beyond those imposed by the conventional signaling system. 

71 
72 
Appendix D. 

Deliverables 

The NAJPTC project produced the following deliverables. 

Software and Document Deliverables 

Current Release 

System Specification Revision (Rev.) 3.3 

Concept of Operations Initial Release for Build 2 

Administrative Contract Data Requirement Lists (CDRL)

A001 Project Management Plan Rev. A 

A002 Project Schedule Initial Release 

B001 Configuration Management Plan Rev. A 

C002 Software Development Plan Rev. A 

C003 Hardware Development and Integration Plan Rev. A 

Software Requirements and Interface Requirements Specification/Interface Design 

Document (IRS/IDD) CDRLs

C006/C011 IRS/IDD Rev. E, CHG 3 

C007-A Software Requirements Specification (SRS)-

POS Rev. E 

C007-B SRS-PTC Office Remote Terminal Server 

(PORTS) Rev. B 

C007-C SRS-PTC Office Common Object Library 

(POCOL) Rev. E 

C007-D SRS-OBC Rev. D, CHG 6 

C007-E SRS-LDS Rev. D 

C007-F SRS-OBD Rev. E 

C007-G SRS-Work Vehicle Rev. C 

C007-H SRS-Field Segment (WIU) Rev. F 

Hardware Requirement Specification (HRS) CDRLs

C008-A HRS-Office Segment Rev. B 

C008-B HRS-Locomotive Segment Rev. E 

C008-C HRS-LDS Rev. A 

C008-D HRS-Work Vehicle Initial Release 

C008-E HRS-Field Segment Rev. D, CHG 1 

Software Design Document (SWDD) CDRLs

C012-A SWDD-POS Rev. B 

C012-B SWDD-PORTS Rev. B 

C012-C SWDD-POCOL Rev. E 

C012-D SWDD-OBC Rev. G 

C012-E SWDD-LDS Rev. E 

C012-F SWDD-OBD Rev. D 

73 
C012-G SWDD-Work Vehicle Rev. A 

C012-H SWDD-Field Segment (WIU) Rev. D 

Hardware Design Document (HDD) CDRLs

C013-A HDD-Office Segment Rev. B 

C013-B HDD-Locomotive Segment Rev. E 

C013-C HDD-LDS Rev. D 

C013-E HDD-Field Segment Rev. D 

Product Safety Plan CDRLs

F002 Product Safety Plan Rev. D 

Testing CDRLs

D001 Contractor Master Test Plan Rev. C 

D002-

A/D003-A 

Developmental Test Phase Test 

Plan/Procedures Rev. H 

D002-

B/D003-B Operational Test Phase Test Plan/Procedures Rev. H 

D004-A Developmental Test Phase Build 1 FAT Test 

Report Rev. B 

D004-B Operational Test Phase Build 1 

Field/Compliance Test Report—Part 1 Initial Release 

D004-C Operational Test Phase Build 1 

Field/Compliance Test Report—Part 2 Rev. A 

Training CDRLs

F003 Training Plan Rev. A 

Other CDRLs

C001-A ATCS Communications Trade Study Rev. A 

C009 Requirements Verification Traceability Matrix Rev. G 

C010 System/Segment Design Document Rev. B 

C014 Version Description Document Initial Release 

E001 Site Survey Instrument (Locomotives) Initial Release 

F001 Maintenance Plan Update Rev. B 

F007 Reliability Analysis Update Rev. B 

F009 Availability Analysis Update Rev. B 

F011 Revenue Service Support Plan Initial Release 

74 
Hardware Deliverables 

Description Qty Unit 

Office Segment 1 Lot 

Locomotive Segment 

Modification kits for UP locomotives SD-40 6 Each 

Modification kits for Amtrak locomotives (P42 type) 16* Each 

Locomotive Segment Spares 1 Lot 

Field Segment 

Modification kits for control 22 Each 

Modification kits for intermediates 33 Each 

Modification kits for electric locked switches 15 Each 

Modification kits for hand operator switches 2 Each 

Modification kits for grade crossings 74 Each 

Modification kits for defect detectors 8 Each 

Field Segment Spares 1 Lot 

Locomotive Segment 

Conversion of kits for up locomotives SD40 to GP60 type 3 Each 

75 
Acronyms 

AAR Association of American Railroads 

AEI Automatic Equipment Identification 

ATC Automatic Train Control 

ATCS Advanced Train Control System 

CAD computer-aided dispatch 

CBTC communication-based train control 

CDRL contract data requirements list 

COTS commercial off-the-shelf 

CSMA carrier sense multiple access 

CTC centralized traffic control 

DGPS Differential Global Positioning System 

DP distributed power 

EIC employee in charge 

EOT end of train 

FRA Federal Railroad Administration 

GPS Global Positioning System 

GCOR General Code of Operating Rules 

IDOT Illinois Department of Transportation 

NAJPTC North American Joint Positive Train Control 

INS inertial navigation system 

LAP link access protocol 

LDS location determination system 

MIS management information system 

OBC onboard computer 

76 
OBD onboard display 

POCOL PTC Office Common Object Library 

POD Point Of Discrimination 

PORTS PTC Office Remote Terminal Server 

POS PTC Office Server 

PSP product safety plan 

PTC positive train control 

RRF Railroad Research Foundation 

RVCCM requirements verification conditions and criteria matrix 

SWDD software design document 

TDMA time division multiple access 

TPM technical performance measure 

TTCI Transportation Technology Center, Inc. 

UP Union Pacific 

WIU wayside interface unit 

77