Railroad Rules, Signaling, Operations:

ABS, Interlocking and TCS/CTC basics

by Carsten S. Lundsten. Created May 12, 2007.


The aim of this document is to give the reader a overview of the basic types of North American railroad signal systems. The examples used in this document are generic, and many variations exist. Eventually separate documents on the different types will be written. Corrections, comments and suggestions for further updates are welcome.

Automatic Block Signaling

In North America "signaled territory" means tracks equipped with an Automatic Block Signaling system (ABS). The ABS ensures basic train safety by

The ABS does not in its basic form include any means for controlling train movements. Controlling train movements still has to be handled by similar means as in unsignaled territory, though the presence of ABS can shift some of the train separation task to the ABS. 

ABS basics

The basic concept of ABS is that the track is divided into Blocks; sections that are protected by Block Signals. A block is the section of line between 2 consecutive block signals. A block signal is a signal that is so controlled by the ABS that the following (somewhat simplified) basic requirements are met:

  1. it cannot permit display a more permissive aspect than "Restricting" or "Stop and Proceed" (i.e. proceed slowly and be prepared to stop for any obstruction) unless all switches in the block are properly aligned for the main track
  2. it cannot display a more permissive aspect than "Restricting" or "Stop and Proceed" unless the block is free of other trains
  3. restrictive signal aspects are displayed so that safe braking distances are ensured if two trains attempt to enter the same block

1st and 2nd requirements address static conditions in the block - the block signal is simply prevented from displaying "Approach" (or better) if a switch is not positively detected to be properly aligned, or if a track is detected to be occupied by a train. The 3rd requirement addresses dynamic conditions, that even if 1st and 2nd requirements are met, it must be ensured that trains are slowed down beforehand, so they can obey the restrictive block signal. This means looking further away than this one block - more about this later. 

Please note that a block signal does not have to display "Stop" if its block is occupied or a switch is not aligned - "Restricting" requires trains to keep a lookout for these conditions.

Safe braking distances

Trains take a long distance to stop - a train going full speed takes over a mile to do a controlled stop. Since signals are rarely visible that far away, it is necessary to warn trains in advance of signals requiring a slow-down or a stop. The common way to do this is to let a signal not only indicate the condition of the first block, but also include information of the following block (and sometimes a third block, depending on speed and local conditions):

Apart from providing a warning distance of at least a braking distance, the ABS must also cope with dynamic issues. It two trains run against each other on the same track, they must be warned at latest when they are their combined stopping distances apart:

Double Track ABS 

The simplest form of ABS is the the double track ABS. On lines with 2 tracks, each track can be assigned a fixed direction of travel, the so called Current of Traffic. Each track will only be signaled for trains moving With the Current of Traffic, while a (rare) movement Against the Current of Traffic will have to have "manual" authorization. For trains moving with the current of traffic the ABS provides the necessary train separation. Lines so equipped are often called Double Track lines. Double track ABS does not provide safety zones against opposing trains.

Single Track ABS 

Single track ABS has the same functions as double track ABS. For trains in the same direction, the requirements for the double track ABS suffice. For opposing trains, however, the ABS must additionally provide a safety zone against head-on collisions:

Train movements are nowadays controlled using radio permissions (most commonly Track Warrant Control), which is also what safeguards against head-on collisions and dispatches trains to meet at sidings. The ABS only protects against trains that came on collision course by mistake. Previously traffic would be controlled by Time Table and Train Order dispatching.

As the ABS only "looks" at the main track, trains in sidings or other secondary tracks do not influence the ABS. The ABS only monitors the switches leading in and out of the main track. The switches are usually hand-thrown or spring switches. There are rules guiding when and how trains may open switches and enter the main track:

There are a number of different ways to implement these requirements, among which the Absolute Permissive Block system is probably the most well known.


An Interlocking Plant (usually just called an Interlocking) is an area with controlled signals and controlled switches, in which signals and switches are interlocked in such a way that

An interlocking typically controls a number of power controlled switches, and allows trains to move from one main track to another, enter or exit sidings, yards etc. In the example below an interlocking controls one end of a yard on a double track mainline, and also controls the junction with a branchline:

In the example below, routes are lined up for an eastbound run straight through the junction, while a westbound is lined up from the mainline into the yard. The controlled switches in these routes are locked in position while the routes are set, and the interlocking blocks conflicting routes and keeps the corresponding signals at "Stop":

When a train has passed the interlocking, the route releases, allowing the tower operator to line a route for another train:

A route can be manually released in case it's not needed. The signal drops to "Stop", but the switches remain locked for a time (often 5 minutes) sufficient to allow an approaching train to come to a safe stop or to occupy the interlocking. The train may have ancountered previous signals indicating a clear route through the interlocking, and may be appraching the interlocking at speed. This Time Locking prevents derailment or collision from the tower operator moving switches before the train is brought to a stop. Some interlockings feature Approach Locking, in which Time Locking only becomes activated if the approaching train is so close that it may have seen (approach) signals to the interlocking.

Interlockings and Traffic Control System

A Traffic Control System (TCS) combines interlockings with ABS and Traffic Locking

Traffic locking locks the direction of travel in a track when an interlocking clears a signal to that track, and it prevents the interlocking in the other end of the track from sending trains onto that track. Traffic locking locks all opposing signals to their most restrictive aspect, and blocks adjacent interlockings from clearing a signal towards that track. Traffic locking stays in place as long as there is a signal cleared to that track, and as long as the track is occupied by a train:

TCS rules are often referred to as Centralized Traffic Control (CTC) rules, but the correct term is TCS (see below about CTC).

Centralized Traffic Control

Centralized Traffic Control (CTC) is a term used to describe a system that allows remote control of a Traffic Control System. CTC allows a single dispatcher to directly control and monitor a long section of railroad, often a whole subdivision. CTC provides the greatest traffic capacity for a line, as the person with the traffic overview is also the person directly controlling the traffic.

CTC consists of ordinary interlockings and ABS with traffic locking as in TCS, with an interlocking often refered to as a Control Point (CP). In the example below an eastbound train is overtaking another to the left, while two westbounds are approaching:

As mentioned above the term CTC is often used to designate the rules of operation for running trains on signal indication alone. The technical requirements for this is the presence of a Traffic Control System, which as mentioned is also part of a CTC system, but it need not be remotely controlled as in CTC.

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Text, Images, HTML: Carsten S. Lundsten.