Hi JC,
The frames are 3 mm. Basically a rectangular box. I deveated from the plans as the original was 12volt with a car air con motor, if memory serves, hung between the frames.
I have mounted the motor vertically in the middle so I could get a battery each side....there was just enough room, I used 2 lengths of 50x50 angle across the frames, & built up from there to mount the motor & gear box. The gear box is a 4" angle grinder head. The motor runs 5000 rpm, the gear box has 4 to 1 reduction, & allows you to get a 90 degree change in a very compact area, from there a 2 to 1 to the axel, then a another chain to the other. The drive between the axel is such that the bottom length of chain is tight when running forward, helps lift it up to give you a bit more clearance if running on ground level track.
I have some photos somewhere of him in bits if you are interested. On a cost bases the batteries I used are marine, I figured that they take a pounding so with the fiberglass matts in them would help, & deep cycle were just that much more expensive again! The controller, I upgraded it last year, should have done so years ago, to a 4QD DNO 5. I recommend that you buy a decent controller from the outset, the original I used worked fine but didn't have regen braking, & as I had quite a few young people driving, & plenty of adults, having to explain that every time & them forgetting could lead to issues!
Cheers Kerrin