I cannot see the parts #'s to determine which diameter curve you are using. O36 curves with a 5" in the center fit on a 48" wide table. Looks like your roughly 4 x 8 has expanded quite a bit.EDITk I see that it's 2 4.5" - so only a 4" growth - 52" will be expensive lumber - wise.
Whichever siding that you are using to park an engine, put a 1 3/8" track right after the divergence (turn-out) to accommodate a simple toggle switch to kill power to center (Hot) rail.