You can run 2 rail stuff on 3 rail track for the most part. It may bounce through switches but you can fix it if its problematic. You dont need dedicated freight cars, just convert a bunch of 3R ones. NWSL makes wheel sets that drop into just about every manufacturer's trucks. The big pain is taking the trucks apart without launching the springs into the 4th dimension. You can also do it by buying pre-done 2-rail trucks (oddly enough, for about the same price) for a lot of brands.
MTH 2R trucks have smaller bolsters than 3R ones (lowers the car, dont need the coupler spacer included with the car)
Atlas trucks usually just screw right onto Lionel cars (and lowers the car)
