I’m starting to think seriously about the drive train for 622. On #10, I built my own gear box, which enabled me to model the full backhead and floor of the cab. That’s all very nice when you take the roof off, but most of the time you can’t see it.
For 622, I’m considering the Northwest Short Line 28:1 Idler gear box kit, which should enable a 12×24 motor to fit horizontally in the boiler. You can just see its purple shape in the lower half of the cab and behind the rear wheel. Painted black, with a crew in place, and some distracting backhead detail, it should disappear.
With a NWSL gear box, I lose the easy opportunity to create a torque arm. So, I’m currently grappling with how to mount the motor. I’m open to suggestions!
Pembroke:87 
I would suggest mounting the worm directly on the motor shaft. You might have to get an adapter kit with new bushings and worm. You won’t need a torque arm then. Just anchor the motor to the chassis with silicone to account for any stress on the shaft. I have done this on some of my locos and they run smoothly with good pulling capacity.
Thanks for the suggestion, Don. I really like the space efficiency of mounting the worm on the motor shaft. Wouldn’t this scheme make the gearbox want to rotate around the motor, thereby lifting one wheel or the other?
Yes, it wants to do that. That’s what the silicone caulk is for, to anchor the motor to the chassis. I’ve always applied the caulk and then run the motor while the caulk cures. This allows the motor to find its best place and prevent any bending or binding of the motor shaft.
Running the motor during curing sounds like a great pro tip!
Another way to do it is screw a torque arm to the gearbox and secure the other end to the motor. Here is how I did it with one loco. http://sandcrr.blogspot.com/2016/06/ormsby-now-in-service.html
This looks like a great approach. I need to get the gearbox and motor into my hands before finalizing the design.