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 booster 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 booster will supply 10 volts, a typical HO scale booster supplies 14 volts and a G scale booster 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.
For controlling live steam locomotives, one would probably use the minimum 7-volt signal.