The armature looked pretty good, but I replaced it anyway. The major wear point was the hole in the phenolic plate that supports the armature on one side of the motor. The end of the armature shaft had opened up considerably. I had a spare MTH Build-A-Loco motor that I had purchased for parts about 10 years ago and replaced the entire side frame, including the phenolic portion. I think I may have replaced the idler gears and the diamond shaped piece that retains them and supports the armature bearing on the other side, many years ago.
I put a lot of time on this 400E-I used to run it in the garden for hours on end.
I have been through a lot with this engine. When it was about 12 years old, I noticed that there was a gap at the bottom of the boiler front. The frame was also bowed down. The frame had actually grown in length about 3/16" of an inch, over time.
I ordered a new frame from MTH then had it custom painted to match (it is not perfect, but pretty close). This 400E was Samhongsa production.
I have two newer 400E's, both by MTH, one about 15 years old, the other about 16-neither one has had a problem with the frame growing, at this point.