Over the summer I was able to attend a steam-up at a local enthusiast's home. Wow! There are some seriously interesting live steam engines out there; available either as kits or ready-to-run. The majority of the equipment at the event was 16mm scale (~1:19 proportion) and ran very well, often at surprising velocity.
16mm scale appears to be the ideal scale to pursue construction of the relatively small War Department equipment - a live steam, 2' gauge Baldwin is actually available in this scale (but I didn't dare ask the price).