The various track diagrams at this Web site are color coded, with different colors representing the different types of signal systems.
Yard Rules - in use on all trackage not part of a main line. Trains proceed prepared to stop within half the range of vision, able to stop short of obstruction ahead.
Track Warrant/Track Permit - written forms authorizing trains to occupy designated lines. Train dispatchers may transmit instructions by radio to crew members, who will write this information on these forms.
Direct Traffic Control - a system where a portion of railroad is divided into "blocks", each of which has a name. Train dispatchers will authorize the occupancy of blocks by naming the blocks authorized.
Train Order - an older written form authorizing train movements, mostly replaced with Track Warrants, Track Permits, and Direct Traffic Control systems.
Timetable Operation - a system where train movements are authorized by being scheduled in the timetable. Any deviations from the timetable (delayed, extra, or annulled trains), would be provided for by train orders. This used to be a standard method of train operation throughout the country, now is quite rare.
Time Spacing - a means of controlling movements of trains in the same direction without automatic block signals. Trains must be spaced at least 10 minutes apart, and trains making unexpected stops must provide protection against following trains.
Manual Block System (MBS) - a means of controlling movements of trains in the same direction without automatic block signals. Sections of railroad would be divided into blocks, which extended between stations or manual interlocking towers attended by operators. Signals would be controlled by the operators. And a train would be permitted to pass a signal, only when the operators at both ends of the block agree that the block is unoccupied. This method is now extremely rare in North America, where staffed stations are too far apart to allow only one train at a time in a block. MBS is more commonly used in Europe, where passenger trains are more frequent and more stations are staffed.
Manual Interlocking - a location where switches and signals are controlled from a nearby tower. Switches and signals are "interlocked", meaning that they cannot be set for conflicting movements. Most traditional manual interlockings have been phased out, in favor of remote control interlockings and Centralized Traffic Control (CTC) installations.
Automatic Block System (ABS) - a modern means of controlling movements of trains in the same direction. Sections of railroad would be divided into blocks, and automatic signals at the entrance of each block would convey the presence of trains ahead. The presence of a train would allow electricity to pass from one rail to the other, enabling the signal equipment to detect a train.
Multiple Track - each track is equipped with signals for a designated direction, or "current of traffic". If necessary to operate trains against current of traffic, one of the methods for unsignaled trackage must be used.
Single Track - signals regulate movements in the same direction, and protect against movements of the opposite direction. But otherwise, movements are regulated by any of the various methods used for unsignaled trackage.
Centralized Traffic Control (CTC) - the most modern method for dispatching trains. A train dispatcher sits at a computer or control panel, and is able to control all switches and signals on a line by remote control. All train movements in both directions are thus authorized strictly by the indication of the signals. And on multiple track lines, any track can be used equally as well in either direction. Some CTC installations are controlled from a manual interlocking tower, instead of by a dispatcher.
Controlled Block System - similar to CTC, with train movements in both directions authorized strictly by signal indication. But movements are controlled by the operators of manual or remote interlockings at the ends of each track segment, and not by a dispatcher. One basic method of providing for bidirectional operation on a track between two interlocking towers, involves a traffic lever at each tower. And each traffic lever must be set for the same direction, for a route to be cleared between the two towers.
On many lines with CTC, remote control interlockings are identified with "CP" for "Controlled Point", and often a number referring to the mile location. This practice probably originated with the New York Central Railroad, but was subsequently adopted by other railroads.
With these older technologies, Automatic Train Stop (ATS) typically would involve an electromagnetic inductor at the location of each signal, which would activate a warning in the cab if the signal is not clear. Automatic Train Control (ATC) would involve cab signals, where coded electrical signals would be received from the rails by sensors at the front of the train. The cab signal would continuously present the track conditions. And when the cab signal changed to a more restrictive indication, a warning would be activated. Most lines with cab signals also had wayside signals, but a few lines strictly used the cab signals, except at interlockings. Wayside signals are necessary at interlockings, to clearly indicate the track conditions there.
In 1922, most railroads in the United States were ordered to equip one subdivision or district with ATS or ATC. The order came from the Interstate Commerce Commission (ICC), which at the time had jurisdiction over these safety issues. In 1924, the ICC ordered most railroads to equip a second subdivision or district with ATS or ATC. A few railroads voluntarily made additional installations. In 1947, the ICC ordered all railroads to install ATS or ATC on all lines where the speed limit was more than 79 mph. Some railroads did equip additional lines with such systems. But many railroads simply lowered the speed limits to 79 mph. At the same time, the railroads were ordered to install block signal systems on lines with passenger speed limits above 59 mph or freight speed limits above 49 mph. And some railroads indeed extended the signal systems, while other railroads lowered the speed limits.
Sometimes the terms ATS and ATC were used interchangeably. Some lines were identified as strictly having Automatic Cab Signals, some of with did not have the capability of stopping a train in an emergency. Another technology is Automatic Speed Control, where a maximum allowable speed is displayed in addition to the cab signal. When the actual speed exceeded the allowable speed, action must then be taken to slow down the train. On main line railroads, Automatic Speed Control generally has existed only in the Northeast part of the US. But cab signals with Automatic Speed Control exists on rapid transit lines, including the CTA (Chicago Transit Authority).