Railroad Rules, Signaling, Operations:

North American Signaling Basics

by Carsten S. Lundsten. Last updated 27-09-1998.


Basic Terms
Authority to Occupy Main Track

     Centralized Traffic Control (CTC)
     Occupancy Permission Systems
     Automatic Block Signaling (ABS)
     Yard Limits (YL)
     Restricted Limits (RL)

     Block Register Territory (BRT)

Basic Terms

First of all it is necessary to look at some basic terms and definitions.

A Main Track is a track that must not be occupied without a permission to do so. Other tracks are Secondary Tracks. A Track used for meeting or passing (overtaking) trains is called a Siding. Sidings can be Main or Secondary tracks.

A Station is a named location on the railroad, listed in the timetable. Europeans please note: A station need not be more than a trackside sign!

Switches (Turnouts) may be Dual Control Switches, i.e. they are operated remotely from CTC. Dual Control Switches may be operated by hand locally but this is mostly done during CTC failures. Spring Switches are spring loaded in their normal position. They can be trailed from the other "reverse" track but needs hand operation if a train is to enter "reverse" track.

Restricted Speed is a speed low enough to enable the train to stop in half the visible range, short of stop signal, other train, wrongly set switch or track defect. Normally limited to 15 or 20 mph, depending on the railroad.

A Permissive Signal is a signal that shows "Stop then Proceed" or "Restricting" as its most restrictive indication. Normally indicated by a number plate and or a "G" plate on the mast. An Absolute Signal is a signal that shows "Stop" as its most restrictive indication. Indicated by absence of number plate on mast.

The signals described in this document are all Block Signals, i.e. they govern trains entering the following Block. A Block is the line between two consecutive Block Signals. Distant Signals are signals on the approach to signaled territory, governing the approach to the first Block Signal. A Distant Signal does not indicate the condition of the track between the Distant Signal and the Block Signal. A Distant Signal is identified by a plate with the letter "D" on the mast.

Authority to Occupy Main Track

Any train needs to have some sort of permission to occupy a main track. The required permission depends on the type of operation in effect for that main track. The type of operation for any main track is listed in the employee timetable, as explained below. The main categories of operation are:
  • Centralized Traffic Control (CTC)
  • An occupancy permission system, either

  •      - Track Warrant Control (TWC)
         - Direct Traffic Control (DTC)
         - Occupancy Control System (OCS)
  • Automatic Block Signaling (ABS), often in conjunction with an occupancy permission system
  • Restricted Limits (RL)



    Track Bulletins may at any time modify the operation type.

    In the descriptions below, the following symbols are used. Please refer text for an explanation of terms:

    Centralized Traffic Control (CTC)

    A train may occupy a main track in CTC territory if it has been permitted to do so by signal indication. This means, that a train may enter a CTC track from another track, or move within the CTC territory, on signal indication alone. A signal indicating it is safe to proceed is thus also the authority to proceed. A CTC track can be used for traffic in both directions, though one direction may be preferred in daily operations.

    There is of course a set of rules to let a train enter CTC territory where a signal is not provided (say, from a spur) and to get trains moving in case of signal failure.

    Signals in CTC territory is a mixture of controlled and automatic signals.

    Operation type CTC:

    Proceeding on signal indication is safe.

    Below is an example of a single track CTC line, which would be listed as CTC in the timetable. The example shows how the dispatcher has authorized train movements by setting up train routes (light green lines). The eastbound train is to enter the first siding and meet the two westbounds. The CTC system only sets up train routes when it is safe to do so, other route requests are stored for later execution (blue lines). Automatic Signals are usually permissive and will allow the train to proceed at Restricted Speed (yellow line). Thus the first westbound will be permitted to proceed further as soon as the eastbound is in the siding. The second westbound automatically has permission to follow the first westbound to the next control point (east end of leftmost siding).

    In this example the Sidings are under CTC, meaning that they are to be regarded as Main Tracks (see discussion below ). Though this is the most common nowadays, it is not always the case.It should be noted that this is not always the case. The siding itself may be a Secondary Track and trains be allowed to enter the siding only on signal indication "Restricting".

    Operation type CTC xMT:

    Proceeding on signal indication is safe.

    CTC xMT simply means that CTC is in effect and that the line has x Main Tracks. Thus the example below is a line that would be listed as CTC 2MT.

    It will be noted that the siding areas in the first CTC example are in fact CTC 2MT sections on a CTC line. Tradition calls for this type of line to be identified as CTC but with a note in the timetable that also the sidings are under CTC. The same goes for sidings in CTC xMT territory. The timetable thus expresses the main characteristics of the line regarding to traffic operation. Extra long sidings, that are really used as a second main track, on a CTC line, will be listed as CTC 2MT sections in between CTC sections.

    Occupancy Permission Systems

    The term Occupancy Permission Systems is a self-made term that I use for a non signal based system that allocates permissions to occupy main track. The oldest and perhaps best known such system is the old timetable and Train Order System. The timetable defined the trains and their normal movements while train orders were issued to modify the timetable defined movements.

    Today the Train Order System is obsolete and has been succeeded by systems permitting train movements by radio. At least 3 such systems are in operation in North America, namely Track Warrant Control (TWC), Direct Traffic Control (DTC) and Occupancy Control System (OCS).

    TWC: A Track Warrant is a permission to occupy main track between two specific points. The points must be clearly identifiable and most often station names are used, though mileposts may also come in handy. Detailed description of Track Warrant Control .

    DTC: The main track in DTC territory is divided into named blocks, for example called "Alturas", "Canby", "Ambrose", "Perez" etc. A train the gets permission to occupy one or more blocks, for example block "Alturas" or blocks "Alturas" through "Perez". Detailed description of Direct Traffic Control .

    OCS: For my further study.... (Please someone enlighten me on this!)

    Though these 3 systems differ, they basically do the same job. In this document I will primarily use TWC as the Occupancy Permission System in the examples, though most is also valid for the other systems. Detailed description of TWC and DTC will follow as my time permits, as these two systems are the ones I know well enough to explain. I hope also to be enlightened on OCS and other(?) Occupancy Permission Systems in the future.....

    Operation type TWC:

    There are no signals.

    Applied in its basic form, an Occupancy Permission System has the sole purpose of ensuring that on any given section of main track there is at no time more than one train. The Occupancy Permission System does not require any signals to accomplish this and may be used alone. This type of operation territory is often referred to as Dark Territory. Below is shown a dark territory in which TWC is used, listed as TWC in the timetable. The traffic situation is equivalent to the CTC example above:

    The eastbound train is to meet the two westbounds at the first siding. The dispatcher has decided that the eastbound is to enter the siding and the westbounds to use the main. The eastbound and the first westbound will therefore both get permission to go as far as the west switch of the siding (illustrated by the dark green line), and the eastbound will be instructed to "clear main track". TWC allows to speed up the meet by issuing another Track Warrant to the westbound permitting it to proceed westwards after the arrival of the eastbound train. Similarly the eastbound may get an early Track Warrant, allowing it to proceed once the second westbound has passed.

    The second westbound can only be issued a Track Warrant permitting it to go as far as the first westbound is known to in the clear (MP XX in the example). As the first westbound proceeds, the second westbound can be issued new Track Warrants. Since issuing a Track Warrant is time consuming this system is clearly not suited for dense traffic.

    Operation type TWC/DT:

    There are no signals.

    The Double Track (DT) variation of the TWC scheme is used to create locations where easy meets may take place. Since all switches in dark territory are hand thrown, trains need to stop to reverse or normalize a switch. Normalizing a switch after a train has left a siding has become a troublesome and time consuming matter after the caboose (and thus the rear brakeman) disappeared from the trains. One often used and obvoius remedy is to install spring switches. Though this eliminates trouble when exiting a siding, it still requires someone to reverse the switch to enter the siding and someone to normalize the switch after the train.

    Alternatively the railroad may decide to have the spring switches lined for separate tracks instead, thereby effectively creating a short section of double track:

    Besides eliminating the need to hand-throw switches entirely, this arrangement also allows the trains to pass the location at more than restricted speed (since both tracks are now main tracks). Still, the rule of at most one train on a given section of main track is valid, since our line now has two main tracks.

    The rulebook states that on double track, trains must keep to the right unless otherwise instructed. So, a track warrant not specifying which track to use, implies use of right track. The Track Warrant may instead state which track to use and thus overrule the right hand running. The designation "DT", instead of "2MT", really only serves to define how the tracks are normally used.

    Automatic Block Signaling

    An Automatic Block Signaling (ABS) system is an automatic signaling system that ensures the protection of following trains traveling in a signaled direction. ABS system signals are as the name suggests always automatic. In some cases special signals can be influenced (released) by action of train crews but ABS system signals are not centrally controlled like controlled CTC signals. ABS system signals are mostly Permissive Signals. Certain signals can be Absolute Signals, please refer descriptions of the different types of ABS systems.

    Operation type ABS/DT:

    Proceeding on signal indication is safe with the current of traffic.

    ABS on Double Track means that each track is only signaled for trains traveling with the current of traffic. The ABS ensures the safety for trains running with the current of traffic (dark green and brown lines). ABS/DT is typically found between sections of CTC or CTC xMT where traffic requirements do not warrant the extra expense of bidirectional signaling. Trains can be safely overtaken on sidings in ABS/DT territory without the need for a formal Occupancy Permission System. Rules, often combined with special Leave Siding signals, ensure that the train in the siding does not enter the main track in a way that will compromise safety (cyan line). Trains on such lines are only informally told to, say, take siding to be overtaken. In some cases the dispatcher may leave it to the trains to decide, reporting their positions on the radio, on the best location for overtaking.

    Operation type TWC/ABS/DT:

    Proceeding on signal indication is not necessarily safe, even with the current of traffic.

    Where the ABS/DT section is very long, it becomes impractical not to have crossovers between the main tracks. Even though these crossovers are hand-thrown, they are invaluable to keeping the traffic moving when track work and other causes necessitates to close one track down. To be able to handle trains running against the current of traffic on parts of the line only, an Occupancy Permission System is introduced as an overlay to the ABS/DT. The trains are in normal operation issued a permission to run through the whole line and as such the situation is similar to the ABS/DT type of operation, including the informal way of instructing trains to take siding.

    TWC on TWC/ABS/DT lines is relaxed to ensuring that either all trains on a given section of main track travel with the current of traffic, or only one train occupies the line.

    Operation type TWC/ABS:

    Proceeding on signal indication is not necessarily safe.

    A single track ABS system is signaled for movements in both directions. Following the definition of the task of an ABS system, a single track ABS system is strictly not required to protect opposing trains(!). In reality most single track ABS systems provide an almost complete protection of opposing movements but some situations may result in a low speed collision. An exception to this is the Absolute Permissive Block (APB) system (see below) which in some implementations provide full safety for opposing trains. Most APB applications do, however, sacrifice a little safety for operational flexibility and therefore also needs an occupance permission system overlaid.

    The purpose of single track ABS its to allow trains to follow each other in close succession. The presense of ABS allows the TWC to be relaxed to ensuring that all trains on a given section of main track travel in the same direction.

    Description of single track ABS system used on the Southern Pacific .

    Operation type ABS (or APB):

    Proceeding on signal indication is safe, but not necessarily wise.

    As mentioned above, some Absolute Permissive Block (APB) systems provide full safety for opposing as well as following trains. An APB system may be technically completely identical to an ABS system, the only difference being that signals at the end of siding areas are absolute (Red means "Stop (and stay)") instead of permissive. It is therefore not necessary to use any Occupancy Permission System with the APB, though the Occupancy Permission System may be a handy tool for the dispatcher to control traffic. Please note that two trains meeting face-to-face at restricted speed, as indicated on the Main Track along the left Siding, is perfectly safe, though generally not very practical...

    Some railroads use only informal dispatching in APB territory and this type of operation is listed as ABS or APB.

    Generic description of APB .

    Yard Limits (YL)

    YL denotes a main track on which movements are to be at Restricted Speed. Trains can therefore occupy main tracks without further authority.

    A train moving within Yard Limits under a Block Signal indication (i.e. in ABS or CTC territory) more favorable than "Approach" does not have to move at Restricted Speed. Trains encountering a Block Signal indicating "Approach" within Yard Limits must slow to Restricted Speed immediately, if possible from that Block Signal.

    The requirement for Restricted Speed after encountering "Approach" is probably an extra precaution to avoid mishaps when the train is moving slower within Yard Limits. Within Yard Limits there is a much higher probability of another train or engine wishing to enter the Main Track in question. Even though these trains must comply with rules for entering ABS or CTC tracks, the risk of misunderstanding each others intentions, and misjudging who's moving under signal indication and who's moving at Restricted Speed is much higher than outside Yard Limits.

    Against the Current of Traffic

    Movement on Double Track within Yard Limits against the Current of Traffic is however not permitted, exept by Track Warrant, Track Bulletin or permission from Yardmaster or other authorized employee. While Restricted Speed should make such a move safe, this rule will in practical terms force trains to move with the Current of Traffic, lowering the probability of mishaps....

    Yard Limits and CTC

    Where the Yard Limits are within CTC territory, CTC rules govern, except the above mentioned rule that trains must slow to restricted speed as soon as possible if encountering a signal indication "Approach" or more restrictive. Since CTC rules givern, no movement may enter a CTC track without permission from the Dispatcher. The permission is typically a signal indication but may also be verbal.

    Restricted Limits (RL)

    Restricted Limits denotes a main track on which all movements are to be at Restricted Speed. There is thus no additional need for a formal system to assign permission to occupy main track.

    Movement on Double Track against the Current of Traffic is however not permitted exept by Track Warrant, Track Bulletin or permission from Yardmaster or other authorized employee. While Restricted Speed should make such a move safe, this rule will in practical terms force trains to move with the Current of Traffic, lowering the probability of mishaps....

    Block Register Territory (BRT)

    Block Register Territory is typically a branch line that will normally only be occupied by one train. Before Block Register Territory may be occupied, the train crew must make an entry in the Block Register with identification of the movement, date, time and name of employee in charge of the movement. The entry is completed after the train has cleared the Block Register Territory by filling in the time the territory was cleared.

    Before making an entry in the Block Register, it must be checked that previous entrien are complete, i.e. that there is no other train in the Block Register Territory. If a second train needs to occupy the Block Register Territory, the first train must be informed of the joint occupation. All movements must then be made at Restricted Speed.

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    Text, HTML: Carsten S. Lundsten.