page_title: Glossary of rail transport terms
section_title: D
hierarchical_section_title: Glossary of rail transport terms / D
context_page_description: Rail terminology is a form of technical terminology. The difference between the American term railroad and the international term railway is the most significant difference in rail terminology. There are also others, due to the parallel development of rail transport systems in different parts of the world.
  Various global terms are presented here; where a term has multiple names, this is indicated. The abbreviation "UIC" refers to standard terms adopted by the International Union of Railways in its official publications and thesaurus.
context_section_description: Dark signal
  A block signal that is displaying no discernible aspect, often due to burned out lamps or local power failure. Most railroads require that a dark signal be treated as displaying its most restrictive aspect (e.g. stop and stay for an absolute signal).
  Dark territory
  A section of track without block signals
  Dead man's handle
  A safety mechanism on a train controller that automatically applies the brake if the driver releases the handle. It is intended to stop a train if the driver is incapacitated. In some forms, this device may be pedal-actuated. See also Dead-man's vigilance device.
  Decapod type
  A steam locomotive with a 2-10-0 wheel arrangement
  Defect detector
  A track side device used to detect various defects such as hotboxes (overheated axle bearings), dragging equipment, leaning cars, overloaded cars, overheight cars, seized (locked) wheels, etc.
  Degraded Operation
  Operation resulting from an unplanned event that prevents the normal delivery of train services
  Demurrage
  A monetary charge levied by a railroad to a customer for excessive delay in loading or unloading cars
  Derail or derailer
  A safety device that derails vehicles that pass it, typically to prevent rolling stock from accidentally entering the mainline from a siding
  Detonator
  A small explosive charge placed on the running rail, which detonates loudly when run over to warn drivers in following trains of an incident ahead. Known as a torpedo in the United States.
  Diamond
  Track that lets a rail line cross another at grade
  Diesel multiple unit (DMU)
  A set of diesel-powered self-propelling passenger rail vehicles able to operate in multiple with other such sets. Such units, especially those consisting of a single vehicle, are sometimes termed railcars.
  Direct traffic control (DTC)
  A system in which train dispatchers communicate directly with train crews via radio to authorize track occupancy in predefined blocks
  Distributed power
  A practice employed to move large trains through the mountains. Consists of the locomotives on the head end, a "swing" (mid-train) helper or two, and pusher locomotive(s) on the rear; today, all units are remotely controlled by the engineer in the lead unit. The power distribution alleviates stress on the couplers and relieves the lead units of the full weight of the train, making it easier to move on grades.
  Ditch lights
  A pair of lights, usually found on modern locomotives, located some distance below and outboard of the main headlight. They may also flash alternately when a locomotive sounds its horn
  Division
  The trackage area under the jurisdiction of a railway superintendent
  Dog or dogspike (India)
  A spike with a slightly altered head shape for easier extraction when the spike has become too loose in the sleeper
  Doodlebug
  A self-powered gasoline-electric passenger car used for small capacity rural commuter service. Also a British Rail Class 153 DMU.
  Double
  If a train has insufficient power to climb a grade and no helpers are available, the crew splits the train into two sections and runs them separately to the top.
  Double heading
  A configuration in which two locomotives are coupled head-to-tail so they can haul a heavy train up a long or steep hill. Currently, double heading (and occasionally triple heading) are used primarily by large passenger trains or as a show for railfans.
  Drag
  A long, heavy freight train moving at low speed
  Dragging equipment detector
  See Defect detector.
  Drawbar
  The part of a coupler that attaches to the frame of the car or locomotive; may be equipped with a pneumatic cushion depending on a freight car's design cargo (e.g. an autorack). Alternately, the pinned double bars coupling a steam locomotive to its tender.
  Driver or engine driver
  The operator of a locomotive
  Driver only operation (DO or DOO)
  Operation of a train by the engineer or driver only. Also known as one person train operation (OPTO)
  Driver
  A wheel in contact with the rail that also propels a locomotive
  Dwarf signal
  A signal light that is considerably smaller and closer to the ground than a high-mast signal; often absolute, and pla
caption: A diesel multiple unit in Poland
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