This is very common with this loco. I had one split a few years ago. The replacement gears Bachmann sends are excellent, and I have had no issues at all since replacing them a few years ago.
To get a replacement, the easiest way I have found, is sending in a request via their online service form. (http://www.bachmanntrains.com/home-u...eform_2010.php
). You may or may not get a reply but I got a package in the mail about a week later. Free.
Takes maybe 10 minutes to swap out the gears. It is self explanatory. I have photos in the Bachmann On30 forum on what the replacement parts look like.
Best of luck!