Great idea 
Is the carriage on bearings?
the carriage is on 6 sealed bearings and axles out off some bmx hubs, the rails are an old curtain pole and there's some laminate flooring in there too.
the only parts i bought were a geared motor that turns at 3rpm at 12volt and a timing belt and timing pulley wheels. all bought from ebay for less than a tenner. bought a battery box and some rechargeable batteries too.
Have you tried using the slider to pull focus?
havent tried that but on the second clip the whole thing jerked a little as the camera got about halfway as i dont think it was seated sturdy enough so i blurred it in final cut and gradually brought it into focus to try and disguise the jerk a little.
