Pat, I've been poking around for any inside the Roundhouse info/photos... None thus far. But, here's my take on your question as followings:
As the major steam loco facility for the LIRR with a "good sized roundhouse" it would seem that all stalls be created equal so as not to require locos already serviced underneath via the work pit having to be backed our into another non-pit' stall to allow another loco access. As far as 'flat concrete' I don't see how the would service/lube the running gear/underbody/trucks/etc. Every photo/model I have seen has all the full repair/service stalls with pits. Yes, I'm sure there are proto exceptions (always 'a prototype' for everything in modeling land!