Try putting a six inch straight edge on the draft plate under the drivers. If the frame is bent it wills show up immediately. This is the sort of appearance of a a locomotive that has done a header to the floor at some point in its life; although the boiler and detail does not look hurt.
It is possible that the boiler structure is so stout that mis-assembly has pulled the frame into a twist, as it might be the weaker link.
Great suggestion for checking a possible bent frame. Fortunately for me the loco doesn't have a bent frame and it doesn't appear to have taken a header like my other brass repair job.
The problem was solved so check the thread later this evening. I'm going to answer these other responses and then address what I found to be causing the problem. I appreciated your interest and following along.