I apologize if this is a duplicate tip.
Lionel has acknowledged continuity issues with the middle rail of some of its FT. While an irritant, the solution, if caught during installation, was to bend the pin to correct a sizing or misalignment issue.
If caught after installation, some members have pinched the two middle rails at the joint - tends to raise one or both rails - while others have soldered a jumper wire across the offending joints.
I just had the problem with several pieces of track on my layout and turned to copper foil tape readily available at stores dealing in leaded glass supplies. I use 1/4" width tape, 01.25 Mil thickness and cut a small piece - about 1/4 to 1/2' in length - to bridge the rail gap. For the less energetic, simply lay the tape across the top of the rail though to hide the tape, just position it on the side of the rail - a small portion will end up on the FT roadbed. To avoid the overhang on the roadbed. simply trim the tape a bit narrower and install on the side of the rail. Only needed on one side. To avoid cutting, simply buy 1/8" wide tape. Cost for 50 feet or so would be in area of $6.
The tape is sold with a removable paper cover backing protecting the conductive adhesive. There are no doubt other uses for the tape.
I began using the tape in the early 60's to improve continuity on 1/32 slot car tracks and in recent years when routing MDF for homemade slot car tracks for the grandkids.