Is it a legit rail, or one of those skateboard rails that you put together? I bought one of those and it sucked. The metal was way too soft. I was able to slide it by sanding it down regularly with course grit sand paper and my electric finishing sander. I also rubbed warm weather ski wax onto the rail. I had to keep doing this to make it work, but my edges would dig in and curl back the metal and I would eat shit.
For any metal rail, I would recommend using ski wax for the appropriate air temp. On the back of the pack of wax it has the temp that that wax works for. Buy the right wax and just rub it on the rail. If you want to really do it right, rub it down with a piece of cork after you rub the wax on.