Denatured alcohol on a cloth. It is also called "stove fuel"- so you can guess: be careful about letting it dry before running trains.
I had a fire on my layout, while using probably too much denatured alcohol on a track cleaning car, when a PW engine threw a spark. Fortunately, I put it out immediately, Now I make sure the track cleaning car is well behind the locomotive, but not near any other cars w/ an electrical pickup. I don't run the cleaning car but once every few months. But more often, I just use a red shop towel and denatured alcohol.
The red shop towel folded 6 -8 times, attached to the end of a flexible stick using a rubber band, takes care of reaching into tunnels.
I try not to use a bright boy or track eraser too much, it will eventually remove the plating on the track rails (unless it is unplated- stainless, brass, or nickel silver).
