I certainly don't want to get into the brouhaha demonstrated by earlier discussion of this subject; I do want to add what I believe is the underlying reason for AC in the first place. The electrical power that comes into almost all homes in the U.S. is alternating current, ie, AC, and is most often in the range of 110 volts. To use this source of power to run model trains requires that it be "stepped-down" to something on the order of 15-16 volts. What follows are the "transformers" of "train-sets". These transformers can be made to step-down the voltage over a fairly narrow range, and to step-up the voltage through the same range merely by rotating a dial back and forth between two physical limits, thus producing an essentially simplistic means of controlling the speed of the locomotive by merely turning the dial. Now to use this same source of electrical power for direct current, ie, DC, requires an additional process, namely "rectification". The term simply means that the electrical current on a pair of leads is always "plus" on one lead and "minus" on the other. That is it no longer alternates rapidly between + and - on each lead. This additional complexity and additional cost is is avoidable if the model locomotives remain set up for AC operation.
