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On a train, the lonng boundary layer builds up so thick that ladders and such are not as bad as they'd be on a truck. The cost and fuel involved in streamlining seem likely to outweigh the benefits, for common freight trains. They get their economy from steel wheels and drafting. It is unfortunate about the mix of flatcars and loaded ones, arising from convenience and destinations. The data on coal cars suggests that folding or inflatable fillers might be the easiest improvement to make.
Trains were designed for steam engines, and attempts to introduce lighter passenger cars have generally foundered on the need to deal with the "buffing loads" of two long trains coupling. A modern re-design could have each car powered by its own electric motors, so that the only couplers needed would be electrical sockets to the generator in the locomotive. The lightweight cars might also be designed as monocoques, smooth all over, with the wheel trucks only exposed from below.
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