If it's enclosed with a tunnel anyways, why bother? [unless it were really fast]
If it went along a no-collision cable, though, that could be handy, especially if it didn't have to be straight horizontal, but could automatically work between two points.
I may give that a try sometime.
In the meantime, this might serve your purposes.
viewtopic.php?f=1&t=17907&p=334158&hilit=accelotron#p334158(you can fiddle with equal opposition to make it a little safer to use)