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.
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.
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:
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.
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:
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 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:
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
switches, and allows trains to move from one main track to another,
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:
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).
Text, Images, HTML: Carsten S. Lundsten.