Because your initial description is ambiguous. We are trying to help but we are just shooting in the dark. If we know what you are actually trying to do it would help. We only know what you have told us.
You seem to be asking for something that moves along a rail with no other connection to anything else. A rail is basically there to restrict the movement not to crawl along. You need something else to supply the push or pull.
If you really want something to crawl along a rail you are going to have to make something, this is not a normal component. The photos you have posted are just the bits that move something along a rail, these bits by themselves do not provide any motive power.