This way if the rail was broken or another train entered the block, any codes would not reach the approaching train and the cab signal would again display Restricting. The codes would be transmitted to the train from the block limit in front of it. The system is failsafe in that the lack of code would display a Restricting signal. The pulse rates are chosen to avoid any one rate being a multiple of another leading to reflected harmonics causing false indications. The codes are measured in pulses per minute and for the 4-aspect PRR system are set at 180 ppm for Clear, 120 ppm for Approach Medium, 75 ppm for Approach and 0 for Restricting. The pulses are detected via induction by a sensor hanging a few inches above the rail before the leading set of wheels. Pulse code cab signals work by sending metered pulses along an existing AC track circuit operating at a chosen carrier frequency. US&S electro-mechanical pulse code generator unit generating 180ppm for a cab signal system Basic operation Due to the effect of interoperability lock in, the 4-aspect PRR cab signal system has become a de facto standard and almost all new cab signaling installations have been of this type or a compatible type. Because all trains running in cab signal territory had to be equipped with cab signals, most locomotives of the aforementioned roads were equipped with cab signal equipment. 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. Over time the PRR installed cab signals over much of its eastern system from Pittsburgh to Philadelphia, New York to Washington. Later, passenger engines were upgraded with speed control which enforced the rulebook speed associated with each cab signal (Clear = No Restriction, Approach Medium = 45 mph, Approach = 30 mph, Restricting = 20 mph). Initially the cab signaling system only acted as a form of automatic train stop where the engineer would have to acknowledge any drop in the cab signal to a more restrictive aspect to prevent the brakes from automatically applying. This new system allowed four signal aspects: Restricting Approach Approach (next signal at) Medium (speed) and Clear. The presence of the carrier alone was not meaningful, no pulsing would still mean a Restricting aspect. The pivotal change was that now it would come on above Restricting merely as a carrier and 1.25 to 3 Hz on-off pulsing of it would be used as a code to convey the aspects. The test installation eliminated wayside block signals, and trains relied solely on cab signals.įor its next installation, on the Northern Central line between Baltimore, MD and Harrisburg, PA in 1926 (1927?), the PRR tested another variation of cab signals which dropped the loop signal and switched to 100 Hz for the track signal. The signals were applied one or both continuously to give Approach or Clear aspects while no signal was a Restricting aspect. This signal was shifted 90 degrees from the other. The pickup would sum the approaching current on each side as it carried on past to the far end of the track. The externally returned ”loop” signal was fed into and out of the mid tap of a resistor across each end of the track circuit. The pickup just ahead of the wheels would sum the approaching current from one side with the returning current on the other. The break-sensing “track” signal was fed down one rail towards the oncoming train and crossed through its wheels, returning in the other rail. The first test installation between Sunbury and Lewistown, PA in 1923 used the tracks as an inductive loop coupled to the locomotive's receiver. The task was assigned to Union Switch and Signal corporation, the PRR's preferred signal supplier. ![]() 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. ![]() In 1922 the Interstate Commerce Commission issued a ruling requiring trains to be equipped with automatic train stop technology if they were to be operated at 80 mph or greater. Pulse code CSS antenna mounted under the pilot of an SRNJ diesel locomotive
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