Madison, did you follow mwbs suggestion of looking up nwsl.com under drive units/carpet drive yet?..the flees aren't ready for release and haven't been for a while{darn it}.
You will note that the carpet drive is really 2 rail friendly, but that hardly limits it to 3 rail useage by using another truck as pick up leads for powering it.
You could get lucky and mesh the gears correctly as you last mentioned, but you could easily not get it right and toss you time and investments away in a few laps...it's a fairly precise thing to set gear mess and gear load correctly...bachmann has been trying to get it right for years{lol....oops}. If money is tight for you, do it right the 1st time even if you have to save for it. This carpet drive is truck hung, meaning you will need to support it via the truck side frames.
Another source sans motor is here-
http://www.protocraft.com/category.cfm?ItemID=168&Categoryid=16
You could mount that verticle with a motor depending on given height or lay it horizontal and use universal couplers to a motor.
If the 2 above don't appeal to you go with using a power truck assembly for the rear truck with the same axle spacing.
I admire your tenacity so far, but the "how do I set my own gears" thing might leave your engine sidelined.
I have used twin magic carpet drives on a G scale{boo-hiss, wrong site, I know} electric engine and it went along very nicely, but don't expect more than 1 spare car attached after that - and you'll want to keep that trailing car light just in case.
