what size are the screws that hold the wheel to the axel, its a large scale two truck shay

if i turn off my switch to the smoke the engine does not run, i replaced the broken front pilot trucks do they have anything to do with the smoke switch, I hooked up the wires exactly like the old ones

I have the dual crossing gate # 461214, but I don't have the wiring instructions, everything works but not sure how to hook it up, it has the two clips that go under the track and a longer grey wire that goes where ?