I found this in my travels across the web. Not sure if it has been posted on here before, but it contains a lot of technical information on various US, and a handful of UK here and there, locomotive classes, mostly steam but a few early electrics, that I haven't seen around. It also contains very detailed drawings of some of them with measurements and dimensions for anyone interested in modeling one of them. The most interesting thing of all might be the dynamometer car data for some of the steam locomotives for those looking to make accurate steam engine specs. Towards the end of each issue it also has railroad news on orders placed by various railroads to give the modern reader an idea of how many locomotives might have been active in that class in the given era. I haven't looked deeper yet, but archive.org might have other years of it. Original is courtesy of the Carnegie Library of Pittsburgh.
https://archive.org/stream/railwaylocomotiv40newy#page/n5/mode/2up
https://archive.org/stream/railwaylocomotiv40newy#page/n5/mode/2up