Mike, I fooled with the LED's in a couple of them repeatedly and never got solid performance. I do solder the tabs on fastrack if they require conductivity. I do that for the Fastrack switches as well, any tab that actually provides a working connection and not just structural support gets soldered.
OR you can 3D print a bumper. 
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As an FYI and update to this thread, I just received a new set of the Lionel lighted bumpers. Box has a 2020 copyright on it, so I'm assuming they are the new and improved version. Thought I'd share a photo of the underside for comparison to the old version that most others likely have. There is definitely wiring that runs up the bumper supports to the LED.
I have them powered on to a short test track at the moment, and so far so good.
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They still didn't include the diode to protect the LED against reverse voltage. 
Another fix, or workaround, if you will. If you like the die cast bumpers and have room for it on an existing layout, you could use the transition piece from Fastrak to tube style track using either the Lionel transition piece and a tubular half straight. Or you can use a Menard's transition and not need to add, or modify, anything.
When I used Fastrack, I used the earthen bumpers, they looked better than the lighted bumpers and were more functional. If I need a light, I just embed it in a section of track.
After I originally replied to this thread ( 3 years ago now ) I tried the fixes mentioned in this thread but found out that even a light impact from a string of cars would dis-lodge or even break one of the legs. I bought a couple of the die-cast lighted bumpers and could not be happier. Just some light trimming and widening with a file ( not much ) and Viola'! Perfect, attractive, semi-scale, and semi-prototypical lighted bumpers. I am not kidding when I state they are set-scale and semi-prototypical. Check out the large squareish, concrete bumpers at the Hoboken train station in N.J.
Now if Jersey Transit painted them fire engine red and put a light on top then bingo! Then If you squinted or forgot your glasses they would look kind of close...
Does this mean that Lionel has finally fixed its lighted bumpers??
I have four lighted Fastrack bumpers and at least two of them were always finnicky. It would be great to hear that Lionel has finally figured out how to consistently power a small red LED permanently mounted on the track. I would've thought that would be a small job to manage after whistle steam and water scoop smoke effects...!
I can't speak for the long term, but I left my new set on for several hours over the weekend and they seemed fine. Actually, I even accidentally knocked the test track they were on off my workbench, and the lights didn't even flicker as far as I could tell (and fortunately nothing broke, either).
