Signal and Train ControlRULES, STANDARDS, AND INSTRUCTIONS (R,S&I)

STC – 2

Automatic Block

Signal Systems

Subpart B

Signal and Train Control - Rules, Standards, and Instructions (R,S&I) governing the installation, inspection, maintenance, and repair of signal and train control systems, devices, and appliances.

Automatic block signaling (ABS) is a railroad communications system that consists of a series of signals that divide a railway line into a series of sections, or "blocks".  The system controls the movement of trains between the blocks using automatic signals.  ABS operation is designed to allow trains operating in the same direction to follow each other in a safe manner without risk of rear end collision.  The introduction of ABS reduced railways' costs and increased their capacity.  Older manual block systems required human operators.  The automatic operation comes from the system's ability to detect whether blocks are occupied or otherwise obstructed, and to convey that information to approaching trains.  The system operates without any outside intervention, unlike more modern traffic control systems that require external control to establish a flow of traffic.

Several systems were implemented as a hybrid of bi-directional ABS and signaling systems with traffic control functions.  Most of these fell along the general heading of absolute permissive block, where a typical bi-directional ABS would be outfitted with signaled passing points, with absolute signals protecting the next segment of single track.  Trains entering the single track segment would be automatically detected at the passing point and if the segment was clear of opposing movements, signaled to proceed.  Trains wishing to enter in the opposite direction would be held until the entire segment was again clear of opposing traffic.  Trains working under such a system would follow the bi-directional ABS rules although without the risk of trains meeting head-on.

ABS is not a method of operation, rather it supplements the method of operation (Train Order, Track Warrant Control, Direct Traffic Control, etc.) by allowing following movements in the same direction, provides broken rail protection, and assists in preventing human errors.

49 CFR 236, Subpart B applies to all signal systems within the broad category of ABS, including systems designated as APB.

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Learning Objectives
        - Understand that these regulations are engineering design standards and must be incorporated into the design of any ABS system.
        - Understand that actions taken during the installation, maintenance, or modification of the system must not negate these design standards.	

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The purpose of an ABS system is to provide limited or full protection to a train from opposing trains and allow for protection from following trains.  To accomplish this purpose the system must be designed and maintained to the standard outlined in Subpart B.

CAUTION

The following presentation was developed to provide general information only. For detailed compliance information please refer to 49 CFR 236 – Rules, Standards and Instructions, Subpart B and the S&TC technical manual.  It is recommended that each employee refer to appropriate manuals and regulations when dealing with any specific issue.

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When in doubt, go to the book!

Information

The FRA Rules, Standards, and Instructions are considered to be the minimum standard allowed for the installation, inspection, maintenance, and repair of signal and train control systems, devices, and appliances.  Each railroad may adopt more stringent standards. Refer to your railroad’s signal standard and instruction manual for further guidance.

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Standards

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236.201 Track-circuit control of signals.

The control circuits for home signal aspects with indications more favorable than "proceed at restricted speed'' shall be controlled automatically by track circuits extending through the entire block.

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This rule requires that aspects of signals with indications more favorable than "Proceed at Restricted Speed" be controlled automatically by track circuits extending through the entire block.  The purpose of this rule is to ensure complete track circuit protect throughout the block.  

Applies to automatic block and traffic control systems.

This slide indicates a ABS system circuited such that it function as an APB system.  This is the most popular form of ABS.

Normally all signal will display a green aspect because the track is clear in both directions.  This slide indicates that there is traffic approaching from the right, hence the red signal for traffic from the left.  The approach signal to the red signal is displaying the appropriate “Approach” indication.

The aspect and indication determine compliance with this rule.  A railroad is in non-compliance if any aspect with an indication more favorable than "Proceed at Restricted Speed" is used even though the speed may be 20 miles per hour or less.  The reason for this is that “Restricted Speed”, depending on the circumstances, may be 1 mph

236.201 Track-circuit control of signals.

Rule 236.708 Block - A length of track of defined limits, the use of which by trains is governed by block signals, cab signals, or both.

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Block lengths for opposing movements

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In APB territory a block from absolute signal to absolute signal for opposing movements or from a signal to its defined limits at end of the system.

As the westbound traffic (from the right) approaches the block it has the appropriate authority to continue into the block per its method of operation (Train Order, Track Warrant, etc.).  Once it has entered the block, all the opposing signal will be set to “Stop”.  This is referred to as “Tumble Down”.

236.201	Track-circuit control of signals.

Rule 236.708 Block - A length of track of defined limits, the use of which by trains is governed by block signals, cab signals, or both.

Block lengths for following movements

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In APB territory the block between stations is subdivided into shorter blocks and intermediate signals are installed to facilitate following movement.

As the train enters the block it occupies a track circuit causing the absolute signal to go red.  When the entire train has passed the East bound signal it will display a proceed aspect.

236.201	Track-circuit control of signals.

Rule 236.708 Block - A length of track of defined limits, the use of which by trains is governed by block signals, cab signals, or both.

Block lengths for following movements

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As the train proceeds through the block and passes the next intermediate signal it will set that signal to stop.  Once the entire train has passed that signal it will allow the absolute signal governing entry to the block to display an approach indication.  The opposing intermediate signal will then display a clear indication.

236.201	Track-circuit control of signals.

Rule 236.708 Block - A length of track of defined limits, the use of which by trains is governed by block signals, cab signals, or both.

Block lengths for following movements

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As the Westbound train continues passed the next intermediate the first intermediate signal will display a yellow aspect and the absolute signal governing entrance into the block will display a green aspect.  Normally, all the opposing signal behind the train would now display green aspects, but as you can see there is more traffic approaching form the East.  Therefore, the exit signal for Eastbound traffic has changed from clear to approach.

236.201	Track-circuit control of signals.

Rule 236.708 Block - A length of track of defined limits, the use of which by trains is governed by block signals, cab signals, or both.

Block lengths for following movements

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The first train continues to move through the block and the second train, assuming it has proper authority, enters the block on a “Clear” signal and causes all the opposing signal to be set at stop.

This sequence of events can only be safely accomplished if track circuits extend through the entire block in order to monitor the train movements through the block.  Hence the requirement of 236.201 that “The control circuits for home signal aspects with indications more favorable than "proceed at restricted speed'' shall be controlled automatically by track circuits extending through the entire block.”

236.202 Signal governing movements over hand-operated switch.

Signal governing movements over hand-operated switch is required to display its most restrictive aspect when the points are not in proper position.

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Signal governing movements over hand‐operated switch in the facing direction shall display its most restrictive aspect when the points are open one‐fourth inch or more.

Signal governing movements over hand‐operated switch in the trailing direction shall display its most restrictive aspect when the points are open  three‐eighths inch or more.

Where a separate aspect is displayed for facing movements over the switch in the normal and in the reverse position, the signal shall display its most restrictive aspect when the switch points are open one‐fourth inch or more from either the normal or reverse position.

The purpose of this standard is to ensure that the switch points are closed properly to allow for the safe movement of a train over the switch and if the switch points aren’t in the proper position any signal governing movement over that switch will display its most restrictive aspect.

236.202 Signal governing movements over hand-operated switch. 
        - This rule requires each switch to be so interconnected with the signal system that when the switch is not in proper position each signal governing movements over the switch will display its most restrictive aspect.
        - Applies to both automatic block and traffic control systems.

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This rule does not apply to spring switches.

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236.203  Hand-operated crossover between main tracks; protection.

This rule requires that hand-operated crossover between main tracks provide protection for train operating on both tracks.

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The purpose of this rule is to provide warning to approaching train when either switch is not locked or not in the proper position or if a car or locomotive is occupying the crossover.

236.203  Hand-operated crossover between main tracks; protection.

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Electric Switch Lock

Facing Point Lock

Switch Circuit Controller

and Track Circuits

The required protection can be provided by: 
          - Track circuits and switch circuit controllers.
          - Facing-point locks on both switches operated from a single lever. 
          - Electric locks on both switches of the crossover.

Applies to both automatic block and traffic control systems.

These requirements apply to crossovers between main track and signaled siding in traffic control territory.

Time or approach locking must be provided for electric locking.

This rule prohibits the use of shunt fouling circuits only to provide protection against the occupancy of the turnouts between the two main tracks.

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236.203  Hand-operated crossover between main tracks; protection.

When switch circuit controllers and track circuits are used all signals must be in their most restrictive state, when either switch is not in proper position, or the crossover is occupied by a train, locomotive, or car. 

Signal governing movement over either switch 

must be in its most restrictive state when:

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Where switch circuit controllers and track circuits are used the signals governing movements over either switch must display their most restrictive aspect when either switch is not in proper position, or the crossover is occupied by a train, locomotive, or car.

236.203  Hand-operated crossover between main tracks; protection.

When facing-point locks are used all signals must be in their most restrictive state when either switch is unlocked.

Signal governing movement over either switch 

must be in its most restrictive state when:

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Signals governing movements over either switch must display their most restrictive aspect where facing-point locks are used when either switch is unlocked. 

236.203  Hand-operated crossover between main tracks; protection.

When using facing point lock control by a single lever all the signals must be at their most restrictive state before they are unlocked.

Signal governing movement over either switch 

must be in its most restrictive state when:

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When using facing point locks controlled by a single lever the signals governing movements over either switch must display their most restrictive aspect before the facing point locks are unlocked.

236.203  Hand-operated crossover between main tracks; protection.

Where electric locks are used, before the electric locking releases.  

Signal governing movement over either switch 

must be in its most restrictive state when:

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Main Track Releasing Circuit

Main Track Releasing Circuit

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Signals governing movements over either switch must display their most restrictive aspect where electric locks are used, before the electric locking is releases.

236.204  Track signaled for movements in both direction, requirements.
        - This rule requires that on track signaled for movements in both directions a train shall cause one or more opposing signals ahead of it to display the most restrictive aspect.  
        - Signals are required to be spaced or arranged to provide stopping distance for opposing trains.
        - In absolute permissive block signaling when a train passes a head block signal it must cause the opposing head block signal to display an aspect not more favorable than "stop."

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The purpose of this rule is to ensure that when a train enters a block that an opposing signal is set to its most restrictive aspect and any opposing train is provided adequate braking distance to prevent a collision.

In APB territory the rule requires that when a train enters the block the opposing absolute signal will be set to “Stop”.

“Headblock” Signal is defined by Part 1.1.1 of the AAR Signal Manual as, "A home signal governing entrance into the block between sidings on single track.  

236.204  Track signaled for movements in both direction, requirements.

Is the distance between signals enough to

provide adequate braking distance?

If not. special circuits may be required!

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Signals are required to be spaced or arranged to provide stopping distance for opposing trains.

236.204  Track signaled for movements in both direction, requirements.

Automatic Block Signal System

Signals passed simultaneously by opposing trains

can pose problems.

Both trains enter the block on “Clear” signal.

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The reason for this is because if two opposing entered the block at the same time they would have green signals indicating the track was clear. 

236.204  Track signaled for movements in both direction, requirements.

Automatic Block Signal System

Signals passed simultaneously by opposing trains

can pose problems.

Both trains are approaching the “Stop” signals

At maximum authorized speed.

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The next signal they encountered would be displaying a “Stop” indication.  Without making special provisions for this situation neither train would have adequate braking distance.

236.204  Track signaled for movements in both direction, requirements.

This is NOT going to be good!

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236.204  Track signaled for movements in both direction, requirements.

Overlap circuits (forces tumble-down) may be  required to prevent possibility of head-on collision.  

Remember: ABS is a safety overlay for the method of operation.

Trains are authorized to move by Train Order, Track Warrant, etc.

ABS assists in preventing human error.

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An over-lap circuit is a track circuit place in front of the head block signal that brings the “Tumble Down” sequence prior to the train entering the block.  The length of this special track circuit must be such that it gives the system enough time to execute the “Tumble Down” sequence.

The ABS signals do not authorize trains to enter the block.  They add protection against human error, detect broken rails, and increase track capacity. 

236.204  Track signaled for movements in both direction, requirements.

The train to the right is held, allowing the other train to advance safely toward a meet. 

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As the first train to enter the block proceeds the second train is held behind the opposing absoluter signal.

236.204  Track signaled for movements in both direction, requirements.

The train to the right is continued to be held, allowing the other train to advance safely toward a switch so the trains can safely pass one another. 

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236.204  Track signaled for movements in both direction, requirements.

The train to the left has now taken siding and successfully met the train to the right, and the train to the right has a green signal for departure.

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Once the first train clears the block the second train can safely proceed.’

236.204  Track signaled for movements in both direction, requirements.

ABS Overlap System, it is different from APB.

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236.204  Track signaled for movements in both direction, requirements.

ABS Overlap System

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236.204  Track signaled for movements in both direction, requirements.

ABS Overlap System

Both trains stopped at red signals.

Collision avoided.

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236.204  Track signaled for movements in both direction, requirements.

Absolute Permissive Block System  (APB)

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Opposing Head Block Signal set to STOP.

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In APB territory a block from absolute signal to absolute signal for opposing movements or from a signal to its defined limits at end of the system.

As the westbound traffic (from the right) approaches the block it has the appropriate authority to continue into the block per its method of operation (Train Order, Track Warrant, etc.).  Once it has entered the block, all the opposing head block or absolute signal will be set to “Stop”.

236.205  Signal control circuits; requirements.

(a) Occupied by a train, locomotive, or car.

Control circuits are required

to be installed so that each

signal will display its most

restrictive aspect when the

block it governs is:

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Control circuits are required to be installed so that each signal will display its most restrictive aspect when the block it governs is occupied by a train, locomotive, or car; 

236.205  Signal control circuits; requirements.

(b)Switch is not in proper position;

Control circuits are required

to be installed so that each

signal will display its most

restrictive aspect when the

block it governs is:

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Control circuits are required to be installed so that each signal will display its most restrictive aspect when a switch is not in proper position.

236.205  Signal control circuits; requirements.

(c) Independently operated derail equipped with switch circuit controller is not in derailing position;

Control circuits are required

to be installed so that each

signal will display its most

restrictive aspect when the

block it governs is:

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Control circuits are required to be installed so that each signal will display its most restrictive aspect when an independently operated derail equipped with switch circuit controller is not in derailing position.

236.205  Signal control circuits; requirements.

(d) track relay is in de-energized position

Control circuits are required

to be installed so that each

signal will display its most

restrictive aspect when the

block it governs is:

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Control circuits are required to be installed so that each signal will display its most restrictive aspect when a track relay is in de-energized position. 

236.205  Signal control circuits; requirements.

(d) Or a device that functions as a track relay is in its most restrictive state; or when a signal control circuit is de-energized.

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Control circuits are required to be installed so that each signal will display its most restrictive aspect when a device that functions as a track relay is in its most restrictive state; or when a signal control circuit is de-energized.

236.205  Signal control circuits; requirements.
        - Applies to both automatic block signal and traffic control systems.
        - A signal must display its most restrictive aspect when any of the conditions listed under (a), (b), (c) or (d) of this rule occur. 
        - This rule is applicable to the design and installation of control circuits.
        - This rule does not require that the most restrictive aspect be a red or stop aspect.

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This rule applies to both automatic block signal and traffic control systems.

A signal must display is most restrictive aspect when any of the conditions listed under (a), (b), (c) or (d) of this rule occur.  However, it is permissible, after the signal's most restrictive aspect has been displayed for such conditions, for a push button, switch, lever or other device to be operated manually by the operator or trainman and an indication not more favorable than "proceed at restricted speed" then be obtained.

This rule is applicable to the design and installation of control circuits. 

This rule does not require that the most restrictive aspect be a red or stop aspect

236.206  Battery or power supply with respect to relay; location.

This rule requires that

the source of energy

be located at the end

of the circuit farthest

from the relay where

open-wire circuit or

common return circuit

is used.

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This rule requires that the source of energy be located at the end of the circuit farthest from the relay where open-wire circuit or common return circuit is used.

Applies to automatic block signal and traffic control systems.  (Does not apply to interlockings.)

236.206  Battery or power supply with respect to relay; location.

This rule prohibits use of loop circuits in vital circuitry.

Vital Control Circuits

Battery

Control Relay

De-energized

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Line wire wrap

ENERGIZED

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This rule prohibits use of loop circuits in vital circuitry.

The reason for this prohibition is that if the line wires become wrapped together it will cause the vital circuitry to be bypassed and the control relay will energize.

236.207  Electric lock on hand-operated switch; control.

	Electric lock on hand-operated switch is prohibited from being unlocked before control circuits of signals governing movement over switch are opened.

Approach or time locking must be provided.

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Electric lock on hand-operated switch is prohibited from being unlocked before control circuits of signals governing movement over switch are opened.

Approach or time locking must be provided.

This rule is applicable only to automatic block signal systems.

There are no requirements for the installation of electric locks in automatic block signal territory.  However, if an electric lock is installed on a hand-operated switch in ABS territory, such electric lock must comply with this rule, including the provision that approach or time locking be provided.  

236.207  Electric lock on hand-operated switch; control.

While time locking or approach locking is required on electric switch locks in ABS territory there are no testing  requirements. Therefore the railroad is not required to make periodic tests of the approach or time locking at these locations.

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The testing requirements for approach and time locking do not apply to ABS systems, therefore the carrier is not required to make periodic tests of the approach or time locking within automatic block signal systems.

S&TC  FUNDAMENTALS

49 CFR 236

Subpart B

Automatic Block Signal Systems

The End

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