While I have done 10 or so steam engines I've only converted a few diesels, a Williams E7 (BlueRail), Weaver RS3 (BlueRail), an Atlas F3 (Deltang Rx65b), and an Atlas GP9 (Deltang Rx65b), so far all have performed great. All use a 9.6v, 2000Mah NiMh battery pack and have 2 motors (except the RS3).
This past week I installed a Deltang Rx in my RK NW2. Everything went smooth, except I broke the die cast chassis in half and one of the side frames fell off due to the truck casting lugs breaking off I'm not sure if I have zinc rot of just clumsy (or both).
Anyway, I fixed the breaks with some JB Weld and super glue and so far they're holding.
Now, for the real purpose of my post.
The E7, RS3, and F3 had 2 motors and I wired them in parallel, good slow speed. I wired the NW2 parallel and it had jack-rabbit starts (from 0-more than I want) and a high top speed, not good for switching.
Tonight I re-wired the motors in series and found that, even though I have to turn the throttle knob 1/2 way (which kinda bothers me but I don't know if there's anything that can be done about it) to get the voltage up enough to move the engine, the slow speed is much better and the top speed is at least half of what it was, both are satisfactory to me now.
The engine came with the MTH 3-rail, DCS PS2 command control system and the motors were wired in parallel (I think) and provided really good, slow speed.
Anyway, I thought it was interesting enough to let everyone know.