I'll put in another plug for Catoctin Mountain and the Johnson's. Genuinely good folks that look out for you. I buy all my engines (and most other things) from them and they are my go-to folks. Granted my school work doesn't allow me to get up to Thurmont as much as I like plus Thurmont is about an hour one way from my home (Potomac MD, Montgomery County, near DC) but I always look forward to visiting. Even if I haven't been up there for weeks, Paul and Marcia always remember me, ask me how my school work is going, working on the "real" RR is, how the family is doing, etc, etc, etc. I've been going there since June 2010 and always appreciate Paul and Marcia.
If you need to spread some payments out or something on an item, Paul will not only let you do that, no questions asked, but also let you take the engine home that day instead of holding it until you've paid in full. Then again, maybe he trusts me a bit too much! .
Engine House Hobbies in Gaithersburg is a good shop as well. Louis is, IMO, the best repair man in the area and Ken takes care of you as well. If you are lucky at EHH, some real trains will come by the windows outside and Kevin, the upholstery store guy next door will come over to chat - he's a hoot!