Matt:
When I lived in Houston from the late '70's to late '90's, there were quite a few train stores. Since my money and I moved, most have closed except for G & G Model Shop in West University Place. Their address is 2522 Times Blvd. I've not had a chance to drop in there my last few visits to Houston, but in past visits, at least, they almost always had something that would make my wallet come out. There may be one or two other train stores in the Houston area that have opened since I lived there, but one of the Houston area Forumites will have to help you with those.
Enjoy the world class bugs and humidity when you are in Houston!
Curt