An engine servicing facility for the JGL

[MasonJar]

I think locomotives were also backed in to the roundhouse so that a minimum of smoke and soot would actually enter the roundhouse, but I may be mistaken on that count. The obvious exception to locomotives being backed in would be the Southern Pacific cab forwards since in reality they were designed to run "backwards."

Steam Locomotives were pulled in forward. There was a large ductwork for lack of a better term. That hung from the ceiling for the exhaust to exit the building Note there is a smoke stack for each track in most cases.
Les

Many round houses had exhaust ductowrk at the front and back. Since the "back" of the roundhouse offered the most space for working (think "wide end of the pie") whichever end of the loco needed work would be put in first. This was often the front end, with the cylinders and many of the big moving parts, but not always. For example, if work was needed on the tender-loco connection, the engine may have been backed in.

Many pictures you see of roundhouses, with all the engine "front ends" poking out were posed specifically for the occasion.

Andrew