If for some very odd reason you happen to have a bit of 15 pound (or 30 pound) tar paper lying around (roofing paper), then a small scrap of that placed between the rails would work well too. It is virtually water and oil proof, and although I have never researched it, it appears to me that on the Lionel tubular track, a tar paper product may be what they used on the bottom of the center rails to insulate it from the metal cross-ties. It is flexible but tough, and "molds" to the surface you place it on.
If any new houses are being built nearby, then just wait until they start putting the shingles down, and you will find scraps of the tarpaper lying on the ground.
Mannyrock
