Hi Gerald --
More about 8-wheel (B-B or Bo-Bo) mechanisms, like the one I'm using on my Limit, Pick & Shoehorn RR critter ...
My chassis is a Life-Like Proto-2000 Alco S-1
... I don't think they're currently in production, but Train World in New York has them listed at $29.99/$39.99 on clearance. Nice, smooth mechanism ... better than the old Athearns. Not sure of the exact minimum radius, but mine will run on 12" radius curves ... and that's probably close to the minimum. If you do a web search for "Proto 2000 Alco S-1" you will find some available.
In that thread, Prof K mentioned some other locos:
-- The old Athearn SW-7 units are a bit hard to find now, but they'll run on 10" radius, maybe less. A bit tricky to convert to DCC if that's an issue.
-- The Bachmann 44-tonner and 70-tonner require roughly 15" minimum radius, but can be modified to run a little tighter ...
The new Bachmann 45-tonner is a compact 8-wheel loco that will run on tight radius.
Russ Haigh (a regular on Carl Arendt's site!) has reported that it will run on 7.5" radius. I'm working on an On30 conversion for this mechanism. Once that's worked out, I'm going to "tear in" to the mechanism ... I "think"
that it might be possible to swap the small wheels for larger ones ... since the axles run THROUGH the wheels into the cranks.
Generally, Bachmann uses 2mm diameter axles ... so it's likely that the wheels can be pulled from the axles (or half-axles, I have to look!) ... and larger wheels with the same axle opening can be fit in place ... it will be necessary to check clearance regarding the mounting of truck side frames, pick-up wipers, etc. to make the final determination. BUT ... I'm very interested in this question, so I'll post some notes (eventually). First, I have to finish my On30 project before I "destroy" the sample mechanism!