HEP of course stands for head-end power. I am stuck between liking the practicalness of putting a generator in a boxcar to feed out-of-the-hardware store elements in a rebuild - and between an image of railroading where lights came from oil and heat came from coal. I know I do not like the experience offered at Stone Mountain GA and Six Flags in IL where the steam train ride is accompanied by a rattling infernal combustion engine and whiffs of diesel smoke. Perhaps it is too expensive, but how about Adlake car lights, and a hot-water circulation system out of a small coal or oil-fired boiler? I found this link with a discussion board about boiler-heating systems, maybe it will be useful.
Olin