As bonus materiel, Trains’ editors had a list compiled of existing roundhouses:
http://www.trains.com/trn/default.aspx?c=a&id=6246
Unfortunately for my friend who compiled the list, he didn’t manage to get them all. I’ve already pointed out four omissions to the editors, and I believe there may be more:
Escanaba, Michigan, ex-Chicago & North Western
Wells, Michigan Escanaba & Lake Superior RR, 8 stalls, still in use by RR minus turntable.
Clarkdale, Arizona, Arizona Central RR/Verde Canyon RR, 8 stalls, in use as car shop by RR (3 stalls), minus turntable
And they added the ex-El Paso & Southwestern one in Tucson that I mentioned to them.
In addition, there may be a roundhouse or two on that list that may have been demolished. They didn’t include the one in White River Junction, Vermont, ex-CV, that burned in late 2008; I understand only a couple walls remain.
Here’s another list from the NRHS, for those who want to cross-compare (the Clarkdale, Arizona one isn’t on either list):
http://www.nrhs.com/historic_structures/roundhouses/rh_data.htm