I'm working to compile a listing of surviving roundhouses in North America for an on-line article. I'm interested in the location, when built, for whom, number of stalls (as built and surviving) and current use. You can send such info to me via PM here or to wayne.laepple(at)gmail(dot)com. Thanks to all in advance.

Wayne,

Start here:
http://www.rrshs.org/Struct.index/strucindex.htm

The Railroad Station Historical Society has made itself useful by setting up their database to not only include all rail stations extant, but also (depending on each individual state and its compilers/maintainers) such structures as roundhouses, engine houses, bridges, tunnels, etc.

It doesn't have them all--for example, the Pennsylvania database, maintained separately, doesn't include roundhouses--but it's a great check list.

Preserved historic bridges, from North Carolina to Minnesota to Nova Scotia, have their own website now. You can search for railroad bridges.
http://www.historicbridges.org/