I concur with John23 and others, that the Atlas/K line/RMT ore cars are the better of the three manufacturers. The Atlas cars started in the 70's as kits. The dimensions have remained consistent, as ore loads I've made fit the same. The MTH ore car is the same width inside, but is slightly longer. The load for the RMT car sits lower. The Lionel car is narrower then the other two.
Don