@Rob English posted:6Wheel trucks (Orig or MTH) will not fit 4 wheel cars without modification of the cars... or the truck. The turning stop stamping was modified on the original 6 Wheel cars due to the larger bolster stamping of the 6 wheel truck.
This really got me to thinking as I had always thought that the 4 and 6 wheel trucks were completely interchangeable. Time for an analysis!
I started by fishing an extra 800 series truck out of my parts bin and carefully removing one of the non-power trucks from my vintage 700 series cars and putting them side by side. As I suspected, the trucks are the same height and use the same wheels and axles.
The turning stop tabs for the 800 series truck and the 6-wheel truck line up, although they seem to be cut off on my cars (and there are no slots for them in the frame). The MTH repro cars that I have do have these tabs intact, inserted into guide slots in the frame, so it seems they may or may not be present on any particular set.
I also have an 8-wheeled set of 700 series cars. They have been very heavily customized by a previous owner who was much more skilled with Bondo than I am (my only addition was to finish wiring up the lights!). I am not 100% sure if these trucks are original but they seem to fit pretty well on the cars' frames.
These 4-wheel 700 series trucks are very tall and have enormous Standard Gauge sized wheels, although they are certainly O Gauge in width. I think these would have looked at home with the 256 which is itself a very tall and impressive locomotive. Interestingly, they have a narrower top section than the 800 series or 700 series trucks. My cars do not have tabs but if they did, they would not fit into the same slots as the 800 series since the diameter is different. The mounting post is meant for a washer and cotter pin instead of an E-clip but seems to be the same diameter and just a tad longer.
What I would conclude is that 700 series cars can have regular height 6 wheel trucks or the taller 4 wheel trucks, with or without turning stop tabs. In any case it appears that modifying the truck (bending or removing tabs) would allow it to fit on either frame since the mounting posts are similar.
