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  • Absolute Block - A block which must not be occupied by more than one train.
  • Absolute Permissive Block - An automatic block signal system in which a train will cause the head block signal governing opposing train movements to display a Stop Signal, while permitting signals for following movements to display other than Stop. These signals will have a number plate.
  • Absolute Signal - A signal at an interlocking which is controlled by the dispatcher. In ATCSMon, absolute signals are controlled or indicated by the "G" mnemonic.
  • ARES - Advanced Railroad Electronics System (utilized only on BNSF)
  • ATCS - Automatic Train Control System
  • Automatic Block Signal - A block signal system in which the use of each block is governed by an automatic block signal, cab signal, or both. These signals will have a number plate.


  • Base Control Point - The station which sends controls to wayside devices (see Mobile Control Points).
  • BCP - See Base Control Point.
  • Block - A length of track with defined limits on which train movements are governed by block signals, block-limit signals or cab signals.
  • Block Signal - A fixed signal, hand signal or verbal block indication in the absence of a fixed signal, at the entrance of a block, to govern the use of that block. This signal will have a number plate.


  • Centralized Traffic Control - A dispatching system that allows the dispatcher to control the switches a signals over many miles of track.
  • Code Line - A pair of wires that carry the logic to control and receive indications from a number of CPs along a given portion of railroad track.
  • Code Line - (circa 1955) Can be visually thought of as a completely automatic telegraph system in which the office machine translates the turning of a lever on the dispatchers panel into a coded control message, which is addressed to the proper field location by it's first few characters. At the field location, the long and short pulses are translated into the desired switch and signal actions and then the field locations sends back an indication the the office that the changes have been made. This is then decoded and displayed on the dispatchers panel.
  • Code Line Protocol Converter - An electronic device that will convert other code line protocols to usually either ATCS message structure or ethernet. These are typically used in the dispatching office, but NS has many installed in the field at the last viable RCL location. These then convert the remaining wire codeline information from points beyond onto the radio at the last viable location. Single locations pumping out about 25 ATCS addresses have been observed, each with a single byte of information. Larger CPs from down the wire may occupy two or more addresses in order to send all of the necessary indications in one-byte chunks. Multiple addresses, belonging to the same location, will always be in ascending numeric order.
  • Coded Track Circuits - An electronic means to convey signal logic information through the rails between adjacent wayside signals or interlockings. It requires that there be no insulated joints in the rail.
  • Controlled Point - A place designated in the timetable where signals are remotely controlled by the dispatcher.
  • Controlled Siding - A siding whose use is governed by signals under the control of the dispatcher.
  • CP - See Controlled Point.
  • Crossover - An arrangement of two turnouts that permit a train to transition between adjacent parallel tracks.
  • CS - See Controlled Siding.
  • CTC - See Centralized Traffic Control.
  • Current of Traffic - The assigned direction of a main track as specified in the timetable.


  • DS - Dispatcher


  • *E* - East or east end of a controlled siding.
  • EE - East end of a controlled siding.
  • EEDT - East end of double track.
  • Electrically Locked Switch - A hand-operated turnout that is equipped with an electrically controlled device which restricts the operation of the turnout to protect mainline train traffic.
  • Electrocode - The brand name for the electronic coded track circuit system offered by Union Switch & Signal Company. Coded track circuits convey local signal logic from one wayside signal to the next. They take the place of the about half of the wires on the open pole line. In order for the electronic coding to travel through the rail, all insulated joints between wayside signals must be removed, which is accomplished by installing Grade Crossing Predictors.


  • Fixed Signal - A signal at a fixed location affecting the movement of a train. These signals may have a number plate.


  • GRS - General Railway Signal Company - A manufacturer of railroad signal systems.
  • Grade Crossing Predictor - A type of overlay audio frequency train detection circuitry that can detect train presence and speed. It is used with electronic coded track circuits because it does not need insulated rail joints.


  • Harmon - Harmon Electronics Company, now owned by General Electric Transportation Systems; a manufacturer of railroad signal and communications equipment.
  • Head Block - The fixed signal at the entrance to a segment of Absolute Permissive Block territory, indicated by a Block Limit Signal.
  • HO - See Holdout Signal.
  • Holdout Signal - A controlled point containing only a signal and no turnouts, diamonds or other appliances. Typically they appear to be an ordinary block signal with no number plate, although they can also be an interlocking that has had all of its turnouts or other appliances removed. In the latter case, there may still be indications of NWK for the missing switches.
  • Home Signal - A fixed signal governing the entrance to an interlocking. This signal will not have a number plate.


  • Insulated Joints - A type of rail joint that electrically isolates the rails on either side of it. They will always be found adjacent to wayside signals or at interlocking limits.
  • Interlocking - A named location with an arrangement of signals, switches, derails, moveable point frogs or moveable bridge components interconnected so that their movements must succeed each other in a prearranged sequence, and for which interlocking rules are in effect to provide for the safe movement of trains. For the purposes of ATCSMon, these are also known as MCPs. The electronics that control the facility are referred to as an interlocker or an interlocking machine, a term carried over from when interlockings were controlled by a complicated mechanical device known as a locking bed. The interlocker can be mechanical, electro-mechanical, or a modern day electronic version referred to as a VHLC (Vital Harmon Logic Controller).





  • Manual Block Signal System - A block system in which the use of each block is governed by a written track warrant and a clear block signal indication.
  • MCP - See Mobile Control Point.
  • Mobile Control Point - The field station which communicates with the Base Control Point to indicate (and in some cases control) local conditions. These can be locomotives, vehicles, or fixed wayside locations (controlled points).


  • *N* - North or north end of a controlled siding.
  • NE - North end of a controlled siding.
  • NEDT - North end of double track.


  • On the Sheet - In days of timetable-and-train-order operation, an "OS" message was transmitted to the dispatcher to announce the arrival of a train at a specific location. Today, the term is used to refer to something being within the limits of an interlocking.
  • OS - See On the Sheet.


  • Pole Line Elimination - A term sometimes associated with the installation of radio code line.
  • Pole Line - The telephone pole and open wire system the railroads use to carry dispatcher phone line, commercial power, signal power, signal logic, code line and any other local circuits that are necessary.
  • Positive Train Control - A method to provide train separation and enforce speeds.
  • Preconditioning - An optional feature of Centralized Traffic Control that will automatically revert the switches and signals at an interlocking to a normal (or other preset) configuration upon the clearing of a train. It will usually be found on lines with heavier traffic volumes.
  • PTC - See Positive Train Control.
  • PTS - Positive Train Separation; see Positive Train Control.



  • Radio Code Line - A generic term that describes any form of signal system communication using two-way radio. There are numerous types of radio code line (RCL), as well as a wide variety of frequencies in use. Often time in this Wiki, as well as on the YahooGroups site, RCL is referred to simply as "ATCS" even though ATCS refers to a specific RCL protocol. Examples of RCL protocols include ATCS, ARES, Genisys RFL and HP-1, among others. ATCS protocols are sometimes implemented over communications media other than radio (e.g., satellite or buried fiber optic) which are inaccessible to ATCSMon. RCL has no correlation to radio control locomotives, the more common "RCL" seen in industry publications.
  • RCL - See Radio Code Line.
  • Rule 251 - Signal indication grants authority for trains to operate with the current of traffic. Movements against the current of traffic must be authorized by the dispatcher and will be governed by Manual Block Rules. This is generally referred to as directional ABS or APB territory.
  • Rule 261 - Signal indication grants authority for trains to operate in either direction on the same track.


  • *S* - South or south end of a controlled siding.
  • SE - South end of a controlled siding.
  • SEDT - South end of double track.


  • TB2 - Track Builder 2
  • TB3 - Track Builder 3
  • TCS - Traffic Control System; see Centralized Traffic Control.
  • Traffic Control System - See Centralized Traffic Control.
  • Turnout - A configuration of track components that permits a train to diverge from one track to another. In ATCSMon, turnouts are controlled and indicated by the "W" mnemonic.


  • US&S - Union Switch & Signal Company is a manufacturer of railroad signal systems.
  • UXO - See Universal Crossover.
  • Universal Crossover - An interlocking with a sufficient number and placement of crossovers to allow a train on any track to transition to any other track. A UXO on a two-track mainline will have at least two crossovers in opposite orientations; a UXO on a three-track mainline will have a minimum of four crossovers with at least two right-handed and two left-handed crossovers between adjacent tracks.


  • VHLC - See Vital Harmon Logic Controller.
  • Vital Harmon Logic Controller - A dual processor, computer-controlled electronic replacement for older style mechanical and elctro-mechanical interlockers. It provides the the ability to throw switches, line routes and set signals according to a pre-defined set of local parameters in a manner that will prevent collisions from occurring. These devices are given the name "vital logic" controllers because both processors need to act on the same instruction and come to the same conclusion before any action is taken. If both processors don't come to the same conclusion then no action is taken and the unit essentially shuts down.


  • *W* - West or west end of a controlled siding.
  • WE - West end of a controlled siding.
  • WEDT - West end of double track.
  • WIU - Wayside interface unit.


  • XO - See Crossover.



-- Main.BrianSwan - 27 Oct 2006