Last night I went back through the TP56 thread from a few years ago. Unless a better idea comes along, I'm leaning towards the original idea in the thread of using a Kato truck with a Kato 11-105 critter motor for a couple of reasons.
https://store.katousa.com/product/gg1-powered-truck-with-drive-shaft-silver-n-scale/
Holy forehead slap, Batman.... that's the best of both worlds. I used a Roco E-unit truck on my TP56 just to get that offset gearbox to leave room for the motor. Ron used a center truck tower and made it work. But that old Roco truck had real issues and I'd never do that again. I'd use the Kato truck but a lot more difficult to fit.
Now, since you very well might have the solution here for both, depends what you want to do with it. If it really is a low-speed switcher, consider putting in a 3V Solarbotics 25:1 GM51A gearhead instead of the 11-105 critter motor, which has relatively low torque and high speed. I've changed all my critter motors out to this in my Climax A's built on Kato critter chassis. You drill out the end of the motor shaft, put in a scrap of .030 rod, and ACC in the universal from the critter. Then shorten the driveshaft with a splice to about as short as you can. I have directions. With a 100-ohm resistor, the draw is so light that the torque load is negligible. You get about 25mph flat out screamin'.
I love these things, cheap, easy to work with, high torque, low current, if you can make it fit.
https://www.solarbotics.com/product/gm15aRon did a great job, but half the problems he had was fitting a motor in with a centered tower. I thought i'd beaten that with an end tower, but the truck quality about drove me bonkers. Gearheads are your friends, but only possible with an end tower.
My other 'ultimate gearhead' was putting a Gizmoszone 5.14:1 gearhead mounted on a Kato 12V critter motor, did that on all my Whitcombs, and two tender-drive steam locomotive conversions, and my own TP56 (below). Sadly, Gizmoszone vaporized from Hong Kong, and any attempts to find the manufacturer of those wonderful little gearheads (Marswell? Somebody else?) failed
Link:
http://www.randgust.com/TP5608.jpgIf I'd done that same approach using a Kato quality truck I never would have sold it.