Here is some more info - not a complete list, but the following RTR cars are from BB kits:
All 34’ hoppers, 40’ quad hopper
50’ flat car (with or without varous loads), 50’ flat car with vans, 40’ flat car (with airplane or boat and other loads), 40 derick car, 40’ plupwood car, 50’ Evans auto loader flat, 85’ flat and the 40’ heavy duty flat car
Bay window caboose, wide vision caboose, western offset cupula caboose
All the reefers except the new 50’ ATSF ice reefers - those are new tooling
40’ chemical tank car, 40’ single and triple dome tank car
250 ton crane and the rotary snow plow - but the plow now comes with a MDC tender.
40’ stock car - ALL the passenger cars
The following cars are from MDC tooling:
Eastern center cupula cabooses, ore cars, 3 bay hoppers, PS2 2003 covered hoppers, 60’ flat cars
A great many of the box cars are MDC tooling and some are new tooling, a few may yet be BB tooling.
I know there are a lot of cars I did not cover, especially more modern stuff that is out my era, but this should give you some idea of how the current RTR line is made up of both older Athearn and MDC tooling as well as new tooling.
And some of the older pieces have recieved big improvements - caboose end railings for example.
Sheldon