Russ,
If your maximum print size is going to stay in the 13x19 range, then I'd seriously consider one of the newer high pixel count, full-frame 35mm digital bodies. Cameras like the Canon 5D MkII and Sony A900 will print stunningly detailed images at 13x19, and have many advantages over 645 MF digital systems. Some of those advantages matter a lot when considering your subject material. MF digital backs are slow (in terms of shots/second) and slow (in available noise-free ISO settings) - two issues which WILL matter to you when shooting steam choo-choos under a variety of conditions. When MF backs are good, they're very, very good, but they don't suit many shooting situations.
I certainly agree that a modern 645 system would work better than an RZ with a digital back adapter, however. I love the RZ, but if I were to shoot with one again, it'd still be with film.
BTW, the winter NN shots I posted some time back were all made with a 5D MkII. The new 5D MkII pushed me back to digital after several years of dis-satisfaction with the technology which had resulted in returning to film shooting almost exclusively. I still shoot film, but now the film camera is mostly reserved for good silver BW. For color, the 5D MkII is now the camera of choice.
93 on the NN
One of these days, I'll get the website updated with images from the last several years... (sigh)
Good luck with your decision,
Scott