I'll agree with the use of the variac. You'll likely only need one to power all 12 of your transformers. Just make sure you don't plug anything else into the 'turned down' power strip. For dead simple and cheap, however a mechanical stop may be the way to go.
A bit more complex route, if you were so inclined, and using engines with can motors, might be to open up the locomotives and add a string of diodes on one of the lines to the motor. this would leave any lighted cars and such to keep getting full power while slowing the locomotive.