Railway Signaling and Operations - more detailed information regarding railroad rules and operating methods can be found at this Web site.
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, and 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. Many 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.
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.