Basic railroad signal terminology
Here is a glossary of railroad signal terminology. Signals are used for protection and control of train traffic. However, there is no national standard or system, so signals used by individual railroads may vary.
Glossary of railroad signal terminology
Absolute signal: A signal whose “stop” indication means “stop and stay.” Usually identified by the absence of a number plate, but may also have a plate displaying the letter “A.”
Advanced Civil Speed Enforcement System (ACSES): The train control system that Amtrak uses on the Northeast Corridor. Amtrak developed the system with Alstom in the 1990s. It was the first functioning positive train control system used in daily service. It is similar to communications-based train control systems used in Europe. The system uses embedded track transponders to locate trains.
Advanced Railroad Electronics System (ARES): A branded, experimental train control system developed by Burlington Northern Railway and Rockwell in the 1970s as an early version of positive train control. Wabtec’s I-ETMS has roots in the ARES system.
Automatic Block System (ABS): A series of consecutive blocks governed by block signals, cab signals, or both, actuated by a train or locomotive, or by certain conditions affecting the use of a block (i.e., an open switch or a broken rail).
Absolute-Permissive Block system (APB): An ABS system for single track railroads, configured to automatically provide absolute signal protection between opposing trains on single track and at passing sidings, but allow following trains to proceed governed by permissive signals.
Approach signal: A fixed signal used in connection with one or more other signals to govern the approach to the other signal or signals. This signal gives a warning or advance notice of the next signals in the series will indicate. (Also called a “distant signal.”)
Block: A length of track between clearly defined limits. The block will be marked with wayside signals, cab signals, or both, to govern its use by trains and locomotives. Block occupancy may also be governed by a dispatcher or other railroad staff, possession of a token (ie.: keys to unlock a gate), or written or verbal orders.(Block lengths vary depending on desired operating speeds and other factors governing the trains’ stopping distance. Typically they range from a half-mile to two miles long.)
Block signal: A fixed signal at the entrance of a block to govern trains and locomotives entering and using that block.
Cab signal: A signal located in the engineman’s cab or compartment indicating a condition affecting the movement of a train or locomotive and used in conjunction with interlocking signals and in conjunction with or instead of block signals.
Centralized Traffic Control (CTC): An interlocked system under which train or locomotive movements are authorized by block signals whose indications supersede the superiority of trains for both opposing and following movements on the same track. CTC systems are controlled by an operator/dispatcher who is to manage the flow of traffic. The operator has control over signals, switches, and other operating devices through the CTC system.
Communications-based train control (CBTC): A specific series of technologies that rely on radio signals, and often embedded track transponders, to locate trains and communicate orders to locomotives based on track conditions, geography, weather, and maintenance-of-way zones. Most positive train control systems used in the U.S. will use some form of CBTC.
Current of traffic: The movement of trains on a main track in one direction specified by the rules.
Dwarf signal: A low home signal.
Fixed signal: A signal of fixed location indicating a condition affecting the movement of a train or engine. (This definition includes such signals as switch, train order, block, and interlocking types, as well as stop signs, slow signs, yard limit signs, and other permanently installed indications.)
Grade signal: A permissive signal used along a steep climb and identified by both a number plate and a plate with the letter “G.” The governing rule is usually that a full-tonnage train climbing the grade may pass a grade signal indicating “stop” without doing so. Grade signals are used to keep heavy trains moving under conditions where the stopping distance is greatly reduced by the hill and restarting would be difficult.
Home signal: A fixed signal at the entrance of a route or block to govern trains or engines entering and using that route or block.
Interlocking: An arrangement of signals, turnouts, and locks with their controls interconnected so as to prevent conflicting movements through any combination of tracks such as junctions, crossings, and crossovers. May be operated manually or automatically, and also by remote control.
Interlocking signals: The fixed signals of an interlocking. Interlocking signals are normally considered absolute and may not be passed when displaying a “stop” indication.
Interoperable Electronic Train Management System (I-ETMS): The branded train control system from Wabtec that all U.S. Class I railroads have installed to meet positive train control requirements. This system relies on global positioning satellites to locate trains. The system is in use on more than 57,000 routes miles.
Manual block system: A block or series of consecutive blocks governed by block signals operated manually, upon information transmitted by telegraph, telephone, or other means of communication.
Medium speed: A speed not exceeding 30 mph.
Permissive signal: A signal whose “stop” indication means “stop and proceed at restricted speed.” Usually identified by a number plate, some permissive signals also have a plate with the letter “P,” indicating that a train may pass the signal indicating stop without stopping but at restricted speed.
Positive train control (PTC): Any combination of electronic and mechanical technologies that will affect a train’s movement, including stopping a train, without input from the onboard train crew. In the U.S., PTC technologies are designed to: prevent train-to-train crashes, enforce speed limits, prevent over-speed derailments, protect track workers, and keep trains from running through a misaligned switch.
Restricted speed: A speed allowing a train or locomotive to stop short of a train, obstruction, or anything that may require speed to be reduced (usually no faster than 15 to 20 mph).
Signal aspect: The appearance of a fixed signal conveying an indication as viewed from the direction of an approaching train, or the appearance of a cab signal conveying an indication as viewed by an observer in the cab. Interpretation of a signal aspect is based on prevailing railroad rules.
Signal indication: The information conveyed by a signal aspect. (A yellow light displayed on a signal would be its aspect; “approach” would be its indication.)
Track circuit: An insulated section of track with (in simplest form) a battery connected to the rails at one end and a relay connected at the other. The relay is normally held closed by the current flowing through the rails. When the rails are “shunted,” connected together, by the wheels of a car or locomotive, the relay opens to indicate that the track section is occupied.