I ran across this recently. Its in frames mode and I don't know how to post individual links to each photo, but its worth a look. When you get to the Train Picture Collection, the left column helps narrow searching down a bit, as the subjects are listed by first letter, so if you're looking for say, Western Pacific, go to the link-with-thumbnails that includes W and you'll see at least a couple hundred WP shots, including in-service shots of some stuff that's preserved today. Some of the pictures are obviously "borrowed" and searching takes some patience, but there are some interesting older photos I haven't seen elsewhere on this site:

http://www.trainpage.com/Rick%27sTrainPictures.html