Hmm....if it is multiple segments, you would have to make 3 sets of path_corners for each way, so 3 for going up and 3 for going back down. You'd have to adjust each so each set is slightly behind the one ahead of it, as the path_corners are where the center of the train will be when it starts/ends. If they're all on the same spot, they wull meet up and bunch together.
With some work and tweaking with the path_corners, I'm sure you can get it to work. They had a similar effect in one of the Opposing Force maps in 'Friendly Fire' and in the Counter-Strike Training Mission, so if you have CS or OpFor, you could take a look at those maps and see how it was done.
-Rabid