Pulse code cab signaling

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.

The externally returned ”loop” signal was fed into and out of the mid tap of a resistor across each end of the track circuit.

Over time the PRR installed cab signals over much of its eastern system from Pittsburgh to Philadelphia, New York to Washington.

This system was then inherited by Conrail and Amtrak and various commuter agencies running on former PRR territory such as SEPTA and New Jersey Transit.

The two most pressing problems were the use of high speed turnouts, which allowed trains to take a diverging route faster than the normal 30 or 45 mph covered by the existing cab signals.

To address the problem and avoid a complete rebuild of the signaling system, impair lower speed service, break backwards compatibility with existing cab signals or place too high a reliance on the human operator, an overlay pulse code system was devised for use on Amtrak's Northeast Corridor.

The 270ppm code does break backwards compatibility with the 4-code system, but is only in use around New York Penn Station as part of a high density signaling upgrade.

The 270ppm code and 60 mph speed were chosen to be compatible with the cab signals installed on the Long Island Rail Road trains that also use Penn Station.

The earliest CDUs consisted of miniature signals of the type visible along the track, back lit by light bulbs.

Modern CDUs on passenger trains are often integrated with the speedometer, as cab signals now serve a speed control function.