I have turned the rails before on a bed knife grinder with good results. Had to do it because replacement rails or a new grinder wasn't an option.
There should be a bolt, or bolts, in the center of each rail pushing up to allow you to straighten them after making a change.
You will need to buy a precision straight edge to make sure they are straight when you are done.
Something like this:
http://www.summitracing.com/parts/PRO-67421/Bret Hart
Equipment Manager
Turning Stone Resort