Well, a properly designed, for the purpose, motor speed control is pretty much what most of the modern electronic train power supplies ARE.
A 'Variac' (autoformer, or a true variable transformer, both produce pure AC (sine wave), just a modified, variable version of the input voltage.
Lamp dimmers and motor speed controls generally produce a modulated square wave or 'chopped' AC wave output. Often this is some form of pulse-width modulation. When the pulses get narrow, the result is a "spike' waveform. This can cause pure hell with an inductor. Inductors do strange things, especially when subjected to square waves and voltage spikes.
The big difference between a lamp dimmer (for purely resistive loads only) and a motor speed control is that the speed control is designed to drive an inductive load (the motor).
You generally can't use a lamp dimmer or speed control with much success ahead of a step down transformer ... the waveforms are not right to 'transform' properly. Also, the transformer may 'kick-back' a voltage spike at the lamp dimmer, and damage it (less likely to happen with a speed control).
And, the common lamp dimmer electronics may not work properly if the step down transform is placed ahead of it.
Either way, while it may work, sort of, it's not a good idea, and may damage either the components or the train. The whole circuit needs to be designed to work together for the desired output.
Dan Mitchell ==========
railrider wrote: