Distance Specified Cutoff

I am writing this post for research for a project at school.

My group and I are building a piece of railroad equipment that we need to travel a specified distance down a railroad track. One of our group members has an arduino board and we are trying to see if this can be used for our system. So far our system is going to be roughly 72 pounds which is where we are having trouble finding a motor that will work. We need the sytem to have an input distance between 18-36 inches and then cut off after it travels that far.

Any help will be greatly appreciated

The easiest solution would probably be a Hall effect type sensor on one axle. For example,
Mount a disk on one axle with slots cut in it and use an optical sensor to count the pulses. The more slots on the disk, the more accurate the distance. The motor would be indipendant if needed and receive the control from arduino. If you mounted the disk on the motor itself, you have basically made a servo motor, and that is what they use to control CNC machines with acruacies down to 0.001mm. Although they are a little more sophisticated, the idea is the same. With a servo type
System, you can set it up to be closed loop as well, this basically means that I'f the motor stalls or jams, the board can "see" it and report an error to you and it will always read the actual position, not where it thinks it is.