Pulse code cab signaling

Pulse code cab signaling is a form of cab signaling technology developed in the United States by the Union Switch and Signal corporation for the Pennsylvania Railroad in the 1920s. The 4-aspect system widely adopted by the PRR and its successor railroads has become the dominant railroad cab signaling system in North America with versions of the technology also being adopted in Europe and rapid transit systems. In its home territory on former PRR successor Conrail owned lines and on railroads operating under the NORAC Rulebook it is known simply as Cab Signaling System or CSS.

History

In 1922 the Interstate Commerce Commission issued a ruling requiring trains to be equipped with automatic train stop technology to operate at 80 mph or greater. The Pennsylvania Railroad decided to use this as an opportunity to implement a signaling technology that could improve both safety and operational efficiency by displaying a signal continuously in the locomotive cab. The task was assigned to Union Switch and Signal corporation, the PRR's preferred signal supplier.

Cab signalling

Cab signalling is a railway safety system that communicates track status information to the cab, crew compartment or driver's compartment of a locomotive, railcar or multiple unit, where the train driver or engine driver can see the information continuously.

The simplest systems display the trackside signal or a simplified set thereof, while more sophisticated systems also display allowable speed, location of nearby trains, and dynamic information about the track ahead. Cab signals can also be part of a more comprehensive train protection system that can automatically apply the brakes and bring the train to a stop if the operator does not respond appropriately to a dangerous condition.

Overview

The main purpose of a signal system is to enforce a safe separation between trains and to stop or slow trains in advance of a restrictive situation. The cab signal system is an improvement over the wayside signal system, where visual signals beside or above the right-of-way govern the movement of trains, as it provides the train operator with a continuous reminder of the last wayside signal or a continuous indication of the state of the track ahead.