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FULL MOVING BLOCK (FMB) CONCEPT AND

REQUIREMENTS SPECIFICATION FOR

RAILROAD OPERATIONS

SUMMARY

The Federal Railroad Administration sponsored 

Transportation Technology Center, Inc., (MxV 

Rail), in collaboration with a rail industry 

stakeholder technical advisory group (TAG), to 

refine concepts and develop interoperable 

requirements for the Full Moving Block (FMB) 

method of train control. The team worked to 

develop an FMB Concept of Operations 

(ConOps) document with a focus on interfacing 

with alternative broken rail and rollout detection 

(ABRRD) system architectures; develop an 

Incremental FMB System and Segment 

Requirements Specification document; develop 

ABRRD system requirements; redline the 

Interoperable Train Control (ITC) OfficeLocomotive Segment Interface Control Document 

(ICD), as well as the ITC Wayside-Locomotive 

Segment ICD, to capture recommended changes; 

develop migration considerations; and conduct a 

preliminary safety analysis. Research was 

conducted from May 2020 to September 2022. 

BACKGROUND

As part of an ongoing program to support higher 

reliability and capacity train control (HRCTC),

three new methods of train control have evolved 

from the current form of ITC Positive Train 

Control (PTC) (i.e., Overlay PTC): 1) Enhanced 

Overlay PTC (EO-PTC), 2) Quasi-Moving Block 

(QMB), and 3) FMB. Of these three train control 

methods, FMB offers the greatest safety, 

capacity and operational gains since it employs 

concepts necessary to approach the minimum 

theoretical headways and the maximum capacity 

on railroad main lines (Vieira, Rosales-Yepez, 

Polivka, & Brosseau, 2022). 

Figure 1 shows an example of the overall FMB 

architecture message flow with two trains in a 

following move. FMB inherits most of its design 

from the existing ITC PTC and QMB

architectures (Kindt, Vieira, & Polivka, In press), 

but employs an alternative method of detecting 

broken rail and rollouts to that of conventional 

track circuits. As presented in this project, FMB 

is essentially QMB integrated with an ABRRD 

system. 

Authority is provided by a non-overlapping 

movement authority known as a PTC Exclusive 

Authority (PTCEA) and broken rail and rollout 

detection is provided by an ABRRD system. The 

team proposed three system architectures 

during this project:

1. Head-of-train (HOT) ABRRD: Onboard 

broken rail and rollout detection that 

interrogates the track ahead of the train

2. End-of-train (EOT) Alternative Broken Rail 

Detection (ABRD): Onboard broken rail 

detection that interrogates the track behind 

the train

3. Wayside ABRRD: Alternative wayside 

broken rail and rollout detection (i.e., 

wayside-based) but without the limitations of 

fixed block track circuit-based detection that 

can cause excess train spacing

None of the EOT-mounted devices reviewed 

during this project could detect rollouts to a 

usable extent. Therefore, the EOT alternative is 

referred to as ABRD rather than ABRRD. If an 

EOT ABRD system is used, supplementary 

technology is required to detect rollouts.
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RESEARCH RESULTS 2 | Page

Figure 1. FMB Components and Key Message Flows

With FMB, the train authorization process 

retains the conventional route creation steps in 

which the computer-aided dispatch (CAD) 

system issues route requests that have 

traditionally been executed by control points or

issued as track warrants. The moving block 

office segment uses the route request 

information to automatically create or update a 

train’s PTCEA (ensuring it does not overlap with 

any other PTCEA) and then sends it to the train 

through the available communication path(s). 

A train’s onboard segment sets its target limits 

based on the PTCEA received from the office, 

along with any other information. As the train 

moves, it automatically releases the track behind 

it by rolling up its PTCEA, using either its 

calculated or detected EOT location. Once the 

office is informed of the rollup, it can then extend 

a following train’s PTCEA. The status of field 

devices is continuously monitored by the 

onboard segment via wayside status messages 

(WSMs) broadcast from wayside interface unit 

(WIU) locations. The status of the track is 

provided by whichever ABRRD solution has 

been adopted, either onboard or by a wayside 

device. 

FMB offers benefits in the areas of safety, 

capacity, and reliability, as detailed in Table 1. 

Table 1. FMB Expected Benefits

Category Expected benefit

Safety Improved broken rail detection 

between closely following trains

Capacity and 

Efficiency 

Near theoretical maximum train 

throughput capacity

Reduction in delays when 

returning to MAS after traffic 

interruptions

ReliabilityMaintainability

Increased pull-apart protection 

when the reported vital end-oftrain indicates greater train length 

than estimated, in which the 

pulled-apart cars are protected 

within the limits of the train’s 

PTCEA

OBJECTIVES

The objectives of the FMB Concept and 

Requirements Specification project were to:

• Develop and document the FMB ConOps 

• Develop and document FMB migration 

considerations 

• Develop and document incremental FMB 

system/segment requirements

• Develop and document an FMB preliminary 

safety analysis
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RESEARCH RESULTS 3 | Page

METHODS

The team met with a TAG, represented by key 

industry stakeholders, to report on project 

progress, make decisions on project-related 

issues, discuss the FMB concepts, and present 

and review the results of technical analyses. The 

effort focuses on: 

• Developing the FMB ConOps 

• Developing the Incremental FMB SystemLevel and Segment-Level Requirements 

Specification 

• Developing the FMB Migration 

Consideration Plan 

• ICD changes

• Preliminary FMB hazard and safety analysis

RESULTS

The team created a FMB ConOps document 

that expanded on concepts developed during

the HRCTC and QMB projects with regard to 

PTCEAs. The document also incorporated the 

proposed ABRRD architectures, functionalities, 

and interfaces.

Both System-Level and Segment-Level 

Incremental Requirements for the FMB method 

of train control were developed. These 

requirements define additions and changes to 

the ITC PTC and QMB systems to implement 

FMB functionality. They are not intended to 

duplicate requirements already addressed by 

existing ITC PTC/QMB specifications. In 

addition, ABRRD requirement specifications 

were developed to support FMB implementation 

by providing a standardized framework for the 

development of ABRRD systems.

The team also developed considerations for 

migrating an existing train control system to 

FMB. Several architecture-dependent migration 

pathways were presented.

Researchers identified necessary changes to the 

ITC PTC Office-Locomotive Segment and PTC 

Wayside-Locomotive Segment ICDs. 

Recommended changes were documented via 

redline markups to the existing AAR standard S9361 and S-9362 ICDs, respectively. Other 

standards not redlined but which will require 

changes to accommodate ABRRD/ABRD related 

device data are S-9202, S-9501, and S-9503

(Association of American Railroads, 2020). 

The safety analysis involved identifying potential 

new or altered hazards introduced by the FMB 

concept. In particular, the team identified any 

hazards that may have been introduced by 

integrating alternative ABRRD systems. No 

major hazard(s) were identified that could not be 

mitigated. Some hazards were identified related 

to the EOT-ABRD interface with the PTC Office 

segment, and the appropriate PTC Office 

segment-based mitigations were presented.

CONCLUSIONS

Researchers worked with an industry TAG to 

develop the concepts, requirements, and 

preliminary safety analysis for the FMB method 

of train control. Per the guidance of the TAG, 

additional burden to the onboard processor was 

kept to a minimum, and messages identified for 

modification were modified in a manner that was 

in keeping with their original intended purposes. 

This guidance, along with other project-related 

research, led to the following major outcomes

from the project: 

• The O-PTC and QMB architectural 

foundations and system requirements can 

be fully leveraged for the implementation of 

FMB, with minimal changes to the 

appropriate ICDs.

• To support FMB, recommended changes to 

existing ITC messages in AAR standards 

Office-Locomotive ICD (S-9361) and 

Wayside-Locomotive ICD (S-9362) were 

documented via redline markups to those 

standards. AAR standards S-9202, S-9501 

and S-9503 were identified as additional 

standards that will require edits to 

accommodate ABRRD/ABRD device 

information in the track database.

• Any of the three alternative broken rail 

detection architectures considered in the 

development of the project can support FMB 

operations.
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RESEARCH RESULTS 4 | Page

• FMB can be implemented as an overlay to 

existing field interlocking systems; however, 

full capacity may not be achievable in some 

specific and typically infrequent operational 

scenarios.

• The need for a standard framework for 

developing ABRRD hardware necessary to 

support migration to FMB was identified. 

Thus, an ABRRD requirement specification 

document was prepared as part of this 

project.

FUTURE ACTION

This project focused on fundamental FMB 

functions and the minimum viable alternative 

broken rail and rollout detection functionality to 

support FMB. Validating FMB requirements is 

planned for a future project. 

REFERENCES

1. Kindt, J., Vieira, P. J., & Polivka, A. (In 

press). Quasi-Moving Block System 

Requirements Development. Federal Railroad 

Administration.

2. The Association of American Railroads

(2020). AAR Manual of Standard and 

Recommended Practice; Sections K1-K6. 

3. Vieira, P., Rosales-Yepez, J., Polivka, A., & 

Brosseau, J. (2022). Higher Reliability and 

Capacity Train Control (Report No. 

DOT/FRA/ORD-22/30). Federal Railroad 

Administration.

ACKNOWLEDGEMENTS

MxV Rail would like to acknowledge 

members of the Association of American 

Railroads (AAR) Train Control, 

Communications, and Operations (TCCO) 

committee and the HRCTC/FMB Technical 

Advisory Group for their contributions to the 

FMB project.

CONTACT

Francesco Bedini Jacobini

Federal Railroad Administration

Office of Research, Development, and 

Technology

1200 New Jersey Avenue, SE

Washington, DC 20590

(202) 493-0800

Francesco.Bedini@dot.gov

Veronica Dorsey

Office of Acquisition Services

Supervisory Contracting Officer

U.S. Department of Transportation

Federal Railroad Administration

(202) 493-4239

Veronica.Dorsey@dot.gov

KEYWORDS

Alternative Broken Rail and Rollout 

Detection (ABRRD), Full-moving block 

(FMB), Quasi-moving block (QMB), 

Interoperable Train Control (ITC), Positive 

Train Control (PTC), communications-based 

train control (CBTC)