Here's a problem I had with a new in the box Lionel Legacy Burlington Northern SD-60 Engine (6-28312). After carefully unpacking and doing an initial oil, lube, and smoke fluid, the first run allowed me to enjoy a scraping sound coming from the rear motor/truck area. Given my recent history with poor build quality on Lionel high-end engines, I was actually expecting something like this. Sorry about the sloppy video quality, but I never set out to make a video on this. I had my iPhone with me and thought I'd capture some of the steps as I went along and when I was able to fix it, I thought it would be useful to someone in a similar situation.
In the end, simply re-seating the gear collar into the sprocket and slightly bending the screw tabs on the motor mount towards the frame allowed the motor to be screwed tightly to the frame and solved the problem.