I just repaired a 6 -28029 Big Boy fro someone. I replaced the smoke unit...reg. Etc, The engine is know smoking.
The only issue is 2 springs that I found floating in the engine. One looks like it could go to the truck stem that goes into the frame rail for left right movement. The other??? There are no picture of the trucks for this engine on the Lionel site. The engine was a eBay special. So, who knows? Anyone, work on one of these tmcc big boys.