The attached PDF document shows, step by step, a way to modify Lionel No. 260 bumpers to convert from incandescent bulb to LED and to fit Lionel FasTrack.
I got the idea from this posting: http://cs.trains.com/ctt/f/95/t/228983.aspx but the photos no longer show there.
I wanted rugged bumpers for a children's layout for a railroad museum and the FasTrack bumpers just looked too fragile. Also, there are reports that the LED electrical connection is very unreliable.
Here is the finished result. It is VERY solidly mounted to the track at all four corners. (Orange colored light is just a camera artifact - it glows red.)