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A train control system has been evaluated in which a remote digital computer is used as the feedback element of a servomechanism. Telemetry signals are required between the train and computer and in the reverse direction, and engineering constraints are placed on the implementation. Signaling requirements are described, and experimental measurements of the electrical noise and ensuing analyses are presented. Equipment using phase and frequency modulation was built and tested, and a serial coding system was tested at typical railway properties. Measurements were made at diminishing power levels in order to determine margins of communication security; error rates approaching 1 in 105are anticipated for an operating train control system.