Gas-electric's were always a custom thing:

There is a long history of modelers cobbling together doodlebugs, usually from passenger car parts. You can expect to need to do the same to some degree. Check and see if Bachmann sells replacement wheel sets, I've had to do that for the first-gen Dash-8's I own. Now they run fine. If you can find a good replacement drive, try to find a non-running one on the cheap. Or just make sure it's a newer Bachmann. The shorter Walthers might be the better choice unless you have broad curves.

Cracked gears

Bachmann AND Walthers models both have cracked gear issues.

I' bought one of the Bachmann ones used ...

... at the LTS recently.  Just always wanted one since first seeing on John Allen's layout.

It's got a slight tic that is visible when it's running slow.  It clicks and you can see the truck lift.  I haven't opened it yet as I have too many other things on the go.  Sound like a split gear? 

LTS had told me something was up with it, hence the bargain price of $50 I paid for it.

Split gears and Doodlebugs..

Dear Bob, Mike,

Maybe I just got lucky, but I have at least 2 active B'mann 'bugs on-roster,
and a few more thru the workbench (On30 and HO mech donors), no cracked-axle gears to report...

...for my current layout build I needed Brill 'bug C&O #9055, and thankfully W&R imported exactly the required paint-scheme/# in brass. Managed to snag one via BrassTrains.com for less-than the cost of a current Rapido diesel, now running a Lok V5 decoder and operates wonderfully... (just gotta nerve myself up to install a non-prototypical backup-headlight on it's tail, as the layout requires it to "shuttle" accross the scene...)

You could consider an old Suydam Brill from BrassTrains at a "decent" price,
then re-mech it with a Stanton drive?
(The Suydam mechs are,...shall we say..."of their era",
but the bodies look decent, esp for RTR "dual control" units)

Mike, yes, that "clicking" and slight "hiccup" in the truck's git-along is absolutely telltale of split--gear syndrome...
(I see a visit to the B'mann Spare Parts website in your future...)

Happy Modelling,
Aim to Improve,
Prof Klyzlr

a few more out there

MTS imported the Brill Model 55 in HO brass; pricey but beautiful. Out in Indiana, Model Railroad Warehouse had etched brass kits for the same Model 55 at one time, but a fire at the plant ended all of that. Don't know how many MRW Brill kits actually made it into the market.