Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Pulse code cab signaling is a form of cab signaling 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 1922 the Interstate Commerce Commission issued a ruling that trains not equipped with some sort of automatic train stop technology would be limited to 80 mph. 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 cab of the locomotive. The task was handed off to the Union Switch and Signal corporation, the PRR's preferred signal supplier.

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