The pieces are not plastic, they are die cast metal, I am not sure of the correct term for them, but I would call them the "Sub Frames" for the power trucks. (2) power trucks to each sub frame, which then had a single pivot connection to the "Main Frame", there are two sub frames per engine. The original sub frames could be warped causing them to bind against the "Main Frame" under frame components, ie air tanks. This was a known problem and MTH made replacements available, I probably changed (12) sets back in the day.
I noticed that the newer rendition of this engine could have a similar problem.
A second binding problem is the clearance inside the shell, between the shell and the can motors, the wiring harness can need to be carefully placed to allow clearance.
You may have to file the sub frame edges to round them off a little also. This can be a tedious, finicky repair, the main frame is sheet metal frame can have some warpage also. You would have to check with MTH for parts, they are the same part for the newer rendition as the original.
Hope this helps, RWDeano
