Like most of their Products during the 1980s, these suffered from a poor grade of plastic used in the drive train that was prone to cracking---even while sitting on the hobby store shelf.
This one was no different but I found a fairly easy fix for the mechanically minded individual with a pin vise and some drill bits.
I ordered the axle sets currently offered on the Bachmann parts page, hoping that they were a drop-in fit. Alas, that was not to be---note the difference in the bearings on the two wheelsets...
Note also that the newer gears (in black) are helical cut for meeting with a worm gear and not cut for the spur gearing of the older locomotive...sigh...more on that later.
Of course, Bachmann couldn't leave well enough alone and made the newer wheels with smaller axles, making a simple swap out of the question. What I ended up doing was carefully enlarging the axle holes in the new gears to fit the older wheels. By stepping up to the next size drill bit rather than going for the whole smack in one shot, I was able to keep the new axle holes properly centered with no wobble...
Note that they also had to make the new gears narrower...a problem overcome with a snug fit and an NMRA wheel gauge for uniformity.
The helical cut on the new gears mesh well enough with the spurs and, since Botch...er...Bachmanns aren't noted for their quiet drives at this market level, the slightly elevated growl isn't any more noticeable over any other locomotive of the period.