Good to see you back, Chip!
I'd have to agree with Mike on the floor being either dirt or wooden blocks, as both would stand-up well to abuse, especially dropped heavy items, such as side rods or wheelsets. Both types of floor would be relatively easy to repair, too...a shovel for the dirt, and removal of damaged blocks, with new ones installed.
Scroll down to the machine shop part in this LINK, as it confirms my recollections that the blocks were installed with the end-grain up, acting as the floor's surface.
Wayne
Likely to be dirt, flagstones, or wood blocks, but if it was "modern" it might be concrete.
Welcome back, Spacemouse!
Mike Lehman
Urbana, IL
Is it dirt? Ballast? Concrete with a lube well?
Chip
Building the Rock Ridge Railroad with the slowest construction crew west of the Pecos.