I have been sucked into a website I found called http://www.shorpy.com/

It is a great, huge collection of vintage photographs, including some great shots of railroads and buildings that make great source materials for modeling.

It is all able to be filtered by key words, such as Railroads

The best thing, is that the images have been scanned in at a very high resolution, which lets you see the details that make all the difference. I highly recommend it, but don't start browsing late at night if you have to get up early the next morning, because you will keep clicking and clicking into the wee hours.