The standard implies that the track voltage can range from +/- 7 to +/- 20 volts. Is this how speed and direction are controlled?
No. During operation, the voltage provided by the command station does not vary. The digital decoder within the locomotive controls speed and direction by varying the amount of power provided to the motor. The voltage range of the 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 place +/- 10 volts on the rails, and a typical HO scale command station will place +/- 14 volts on the rails and a typical G gauge command station will place 20 volts on the rails. If you were controlling live steam locomotives, you would probably use the minimum +/- 7 volt signal. The +/- part is because the signal is mirrored around zero volts.
