Johnbeere -- thank you for those scans, it will help a lot.
bob2 -- I appreciate your insight (as always). And you are right, it's a hobby. This is something I've wanted to try for a long time.
prrjim -- all very good points. I have a mix of Atlas switches, the 2 piece Roco switches and Signature Switches. There are no clunks on the signature switches. They are really very nicely made. And while the Roco switches aren't in the same league with the signature switches, the engines don't make the clunk of them either. It's on 2 of the Atlas switches where I suspect the gap is a little bigger. I'm making a change to the mainline that will replace one of the offenders with a signature switch. But I think it is also what you said about balance. The three brass engines I have go across all the turnouts with minimal noise. They are a Sunset decapod, a Sunset Mikado and a Williams Mikado. I have two converted Lionel engines (2-8-2 and 4-8-2) and the All Nation 4-6-0 that are diecast and a lot heavier. The Lionel 4-8-2 and the All Nation are the two that really make the noise. They don't derail, so it isn't a problem operationally.
Given what has been said, I suspect that 90% of this is track work and not the engine.
Actually the sprung drivers were a secondary thing. I never liked how the old open frame motor projects out of the cab of the All Nation engine. So when I bought this mechanism, the plan was to build it with a NWSL gearbox and shorter drive train and shorter motor. I picked the mechanism up pretty cheap since a lot of the envelopes were open and there were essentially two boxes of mixed parts and no instructions. Luckily, the valve gear parts were still sealed in their envelope.
Thanks everyone. I'll see how this goes.