Kurt Bell, the longtime head of the Railroad Museum of Pennsylvania, transferred out there some years ago, and since then they've had a great reputation--he's very knowledgeable and helpful to rail historians. Service felt comparable to going to the Hagley; they have finding aids and will pull material more or less on demand, as I recall. There are fees for reproduction of images in publications (less for nonprofit than commercial), so do be aware of that if you're collecting images for an article.