Many of you likely know this, but it is worth repeating. Google has hundreds of free e-books that cover all sorts of topics including railways. I picked up a history of the Midland, and the same for the Great Western yesterday evening. Search in google for your favorite railway and click books. When it gives you the list, click 'Free Google e-books'. Most of what they offer free is from the 1800s, but that's when it all started!
More there than I've got time to read.
Fascinating reading about our (US) Civil War written a few years after the fact.