FAQ:NMRA Standards State Track Voltages Range from 7 to 20 volts. How are Speed and Direction Controlled?
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During operation, the voltage provided by the command station does not vary.
The DCC multifunction decoder within the locomotive controls speed and direction by varying the amount of power provided to the motor. The voltage of the DCC signal described in the standard is to allow for the different power needs of the various scales. For example, a typical Z scale command station will supply 10 volts, a typical HO scale command station supplies 14 volts and a G scale command station will put 20 volts on the rails.
The NMRA DCC Standard allows for a range of voltages on the track, as well as minimum and maximum voltages that a multifunction decoder must tolerate.
If you were controlling live steam locomotives, you would probably use the minimum 7 volt signal.