I have both 72' streamliners and 72' heavyweights. The exterior looks like MTH's with the exception of the lights (bayonet in the floor) and the window strips. It is similar with the heavyweights. Those look nearly the same as Lionel and MTH. The differences are the lights, window strips, and trucks that do not have brake shoes on the outside of the outer wheels (like MTH used to do).
Both run just like the others, so there are no issues there.
As for upgrading the interiors, I updated all my heavyweights by putting in LED lighting, plastic windows, and interiors. They look very good now and match the MTH ones I have in my consist. The Streamliners can be done the same way. For both you would just have to bend down the bayonet mounts to get the interiors to fit.
I can post pictures if you want me to (I would have to take them to load, but that is easy to do).
