Hey Stefan. Thank you for the reminder about the Tracks Ahead video series. I remember catching those from time to time on PBS, but haven't watched one in years. Another good source of reference photos is Morning Sun books. Their books cover a wide range of railroads and eras. They are mostly color picture books with good captioning. But, they have pics that I haven't seen elsewhere. Books are hardcover and run around the $60 USD price. They also have digital reprints available, and some titles that are only available in digital form.
https://morningsunbooks.com/Another source for more modern reference material is Withers Publishing. Besides producing the magazine Diesel Era, they produce a number locomotive books that are either railroad specific, builder specific, or model specific. Besides excellent reference photos, the authors go into more detail about the models, often explaining history and differences between phases.
https://www.dieselera.com/booksIf you're interest is steam / transition era freight cars and passenger cars, check out Railway Prototype Cyclopedia. These soft cover books go into great detail about a specific car, and often include plans and lots of photos. Unfortunately, the publisher has stopped producing these books, but you can still find them online and at some train specific hobby shops. I have almost the entire series and highly recommend them. So if you want to go "Searchlight Sim" or "Mainlines" on a USRA single sheathed boxcar or a war emergency gondola... these titles for you.
Lastly, Four Ways West books produces a number of hard cover books that are mostly railroad specific titles. Depending on the title, there can be more text explaining the topic, but seem to be more color picture books. As with Morning Sun, they seem to have pictures that I haven't seen elsewhere.
http://fourwayswest.com/books/Lastly, it should be noted that railroad book titles are often limited runs, and titles have a tendency to go "out of print". So if you do come across a title that you like and you think will have value to your modeling (virtual and/or actual), then I would suggest picking up the title while you can. If you have a used book store in your area, periodically check in with them to see if they are received any railroad titles. Every now and then they will get something in... usually from a trade or an estate sale. I picked up a title on Southern Pacific steam locos for $40 USD that is long out of print, and if you find a used copy at a hobby shop... will often go for $100+ USD. In my instance, the dust jacket was worn and had tears, but the interior pages were in excellent shape. Some train specific hobby shops will also sell used books. Last time I was in Des Plaines Hobbies in Des Plaines, IL, they had a large number of used titles available.