My suggestion is not to use any rail etc. If all you want is about 4' of track to run back and forth I can think of several ways. Take a 2 x 10 or 12 and 2 - 1/4" x 1/2 " strips of hot roll. Screw the strips down as your rail 3 1/2" apart. Now you have a test track that has several advantages. One you can pick it up and lean it against a wall out of the way, two your engine will not drop down between 5/8" rails causing potential grief, Clamp it two a couple of "L" brackets to the wall and you have a storage shelf.
The other approach that I have used is to make up a ball bearing type of mount so the engine stays in one place and you can test it forward backward without any movement, see photos. It is for a 4-6-6T.
Bob