I would say Blender. You can get it at Blender.org and there are tutorials here:
http://forums.auran.com/trainz/showthread.php?96642-New-Blender-Tutorials-for-v2-65-and-up
Kind of a little "gotcha" that I want to give you a head's up about: When you create a texture, be sure to click the Mapping arrow and select "UV Mapping" instead of the default, "Generated." You'll see what I mean if you get the program and do some tutorials. Blender is actually pretty easy and fun for buildings, once you grasp the basics, but those basics take a bit of learning and work to get down pat.