**Training Aid**

**S&TC-2-2: Instructor Notes**

**Automatic Block Signal Systems**

**Slide 1** – 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.

**Slide 2** – 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.

**Slide 3** – When in doubt, go to the book!

**Slide 4** – As information.

**Slide 5** – Standards

**Slide 6** – 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 an 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

**Slide 7** – 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”.

**Slide 8** – 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.

**Slide 9** – 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.

**Slide 10** – 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 from the East.  Therefore, the exit signal for eastbound traffic has changed from clear to approach.

**Slide 11** – 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.”

**Slide 12** – Signal governing movements over handoperated switch in the facing direction shall display its most restrictive aspect when the points are open onefourth inch or more.

Signal governing movements over handoperated switch in the trailing direction shall display its most restrictive aspect when the points are open threeeighths 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 onefourth 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.

**Slide 13** – This rule does not apply to spring switches.

**Slide 14** – 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.

**Slide 15** – 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.

**Slide 16** – 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.

**Slide 17** – Signals governing movements over either switch must display their most restrictive aspect where facing-point locks are used when either switch is unlocked. 

**Slide 18** – 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.

**Slide 19** – Signals governing movements over either switch must display their most restrictive aspect where electric locks are used, before the electric locking is releases.

**Slide 20** – 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.  

**Slide 21** – Signals are required to be spaced or arranged to provide stopping distance for opposing trains.

*.*

**Slide 22** – 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. 

**Slide 23** – 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.

**Slide 24** – This is not going to be good!

**Slide 25** – 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. 

**Slide 26** – As the first train to enter the block proceeds the second train is held behind the opposing absoluter signal.

**Slide 27** – Train is held.

**Slide 28** – Once the first train clears the block the second train can safely proceed.

**Slide 29** – ABS Overlap System, it is different from APB.

**Slide 30** – ABS Overlap System

**Slide 31** – Both trains stopped at red signals.  Collision avoided.

**Slide 32**– 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”.

**Slide 33** – 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; 

**Slide 34** – 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.

**Slide 35** – 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.

**Slide 36** – 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. 

**Slide 37** – 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.

**Slide 38** – 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

**Slide 39** – 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.)

**Slide 40** – 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.

**Slide 41** – 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.  

**Slide 42** – 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.

**Slide 43** – End