i know that there was a long thread some months ago regarding the poor performance of the fastrack lighted bumpers.
I had ordered some replacement red led's from lionel last year thinking that the longer leads on the replacements would be the answer compared to the shorts stubs on the included leds.
However, it was still the same issue - flickering.
In frustration, i pushed the led down into the bumper very firmly and that jammed the leads into the sockets and seemed to provide solid electrical contact and no flickering.