hi,
Here you have a simple and interesting project to develop a automatic driver for your Scalextric/slot game. It uses an analog output, and power driver base on TRT tip120.
To make it easier you can use the arduino input-output shield compliant with S4A and scratch. (http://scratch-io.wikispaces.com)

Please, see the video on following link:
video

Could you beat the machine? I can´t