This short video shows a very simple way to motorize a tripod track dolly using friction between two wheels. In the video below i've modded the Cinerails Snaptrack kit.
The main parts are a simple DC motor and Speed Controller from Servo City, and small venom 11V 3S Lipo battery. I mounted the Speed Controller up top so I don't need to bend over to change speed or direction. I mounted the motor just over one of the track wheels, and used washers to get the right amount of spacing.
Unfortunately these were spare parts laying around my house, so I don't have the exact part numbers. While I work on getting that together, here's a few links to get you started. I'm sure If you contact ServoCity.com they can probably help you out with an entire kit.