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Control Points

This page was last updated on September 27, 2026.

(Return To The Union Pacific Control Points Page)

Mark Hemphill Comments

A Control Point (abbreviated as CP, or C.P.) is any on a railroad's main track where a person working remotely or at that location can operate signals and switches to change a train's route, deliver a signal indication to a train that provides authority to occupy the main track or certain side tracks ahead, or both actions simultaneously. Control points are most often established at turnouts, crossings of two or more railway lines at grade, and at hold signals – the latter a location where a train dispatcher wishes to hold a train on the main track short of a yard or terminal to allow trains to switch in the terminal unimpeded. Intermediate signals between sidings or crossovers are not control points; they operate automatically based upon track conditions and train occupancies ahead. Also not a control points are automatic interlockings, which are "first-come, first-serve" for whichever route's train arrives first. Control points require human intervention in order to change their condition. For that reason, interlockings that are control points are called manual interlockings – the "manual" has nothing to do with the mechanism of changing a signal's indication or throwing a switch, only that a human must issue the command to the mechanism.

All possible entrances to a control point are protected by signals. For example, a siding switch from a main track has three entrances; a crossover between two main tracks had four entrances, and two main tracks crossing two main tracks has eight entrances. A hold signal has only two entrances. All signals are interlocked and cannot provide an indication better than Stop unless no conflicts are detected, and will also not clear if any switch within the control point is misaligned or improperly aligned. Control point signals always are "absolute" signals, which are defined as a signals that convey authority to occupy the main track (or controlled siding) ahead. Intermediate signals carry no authority; a train's authority to occupy the main track is provided by timetable rule, track warrant, train order, or similar, or a controlled signal that precedes the automatic signal. The Denver & Rio Grande Western and a few other railways referred to their absolute signals as positive signals, functionally identical to an absolute signal.

Control Points are designated by name and milepost in an employee timetable or other written instruction provided to all train crews, track maintainers, and signal maintainers that are assigned to duty on that main track, and identified in the field by name. Originally, control points were merely designated by timetable station name; it was up to a train crew or maintainer to understand if they were looking at "East Dumble" or "West Dumble". In recent years, many railways have begun prefacing the station name in the field with CP, and adding "East" or "West" (or North and South) to reduce the risk of a trainman or maintainer confusing the two, and often the milepost, too. Thus what was once a simple field designation such as "Summit" has become "CP 422.8 East Summit".

Control Points can be controlled locally, from a wayside control box, or from a locomotive cab using radio commands. The control point is typically interlocked with track circuits on all tracks leading away from the absolute signal at the control point's entrance that will not operate a switch or provide a signal indication better than stop if the circuits detect a conflicting movement or track occupancy ahead.

Control Points are a feature of Centralized Traffic Control systems – every location with controllable signals on a CTC-equipped railway is a Control Point. Until about the 1980s, interlocking operating rules were not always the same as CTC operating rules, but with the development of modern rule books such as the General Code of Operating Rules, the terms "interlocking" and "control point" have become synonymous. Control points may be an island within an railway territory otherwise dispatched by Track Warrants, Direct Traffic Control authorities, or similar. Remotely operated drawbridges are a common example of a control point (spanning the entire bridge) in the middle of otherwise non-signaled territory.

Control Point equipment includes the wayside signals, which may be dwarf or mast-type, or mounted on a cantilever or bridge, power-operated switches, if present (none at a hold signal), one or more instrument houses (also called signal bungalows) that contain the power supply to the switches and signals, microprocessor or relay-based logic for the interlocking features among the signals and switches, battery backup in case of power outages and a battery charger, and an interface to the control communications system, which may be hard-wire, through the rails, microwave, fiber-optic, or VHF radio. A utility electric power drop is typically provided; some control points in remote areas are solar-powered. Some control points feature alerting systems to warn maintainers of an approaching train. Ancillary cases and cabinets hold equipment such as track shunts and variable resistors to reduce cable runs back to the instrument house(s). Railways with a Positive Train Control or Cab Signal overlay of their signaling system house the necessary wayside equipment in the same instrument house, along with the PTC equipment's data radio.

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