Well, I have one going as well, although not inspired by Robin's recent work. I started this one ages ago and stalled out (as most of my projects seem to do) when either I ran out of materials or another project got started. I cannot even remember which it was.
Mine is 48 feet long, not quite long enough for a C-16, but will fit a Bachmann 2-6-0. My premise is that once the railway got larger locos, they built a newer, bigger stone roundhouse for them, and leased out the old building to a certain Gauge member, who now runs a machining and manufacturing business out of this older single stall enginehouse. I hope to leave the one near wall unfinished, as I hope to eventually have this building's interior detailed.
I also hope to be able to transfer this building to the logging section (if it ever gets done...) on my modular On30 layout. It should fit a Climax or Shay quite nicely.
Here's where I am so far.