Here is a simple timed DC throttle that has controls for the length of run, accelerate/ decelerate and the maximum train speed. The circuit also has automatic current limiting.
An ideal power supply fo this circuit would be a AC wall adapter. This would likely be less expensive than building a power supply from scratch. The 7812 voltage regulator would not be needed if an adapter with a built in regulator is used.
http://home.cogeco.ca/~rpaisley4/xTimedThrottle.html