I have 6 locomotives now with Kadee remote couplers. I've been using Arduino Pro Minis to drive them. The Arduino Servo library works well. I have found, though, that I need to cut the power to the servos (using an N-channel MOSFET in the ground) when not in use, or the motor noise makes them chatter when the engine is moving fast. It also saves power, of course.
In my case, I'm decoding the DCC function packets in the Arudino (AirWire), but you could easily use a logic input to drive it.
Parts cost around 5 bucks.