Another FANTASTIC resource is "Mitchell's Models and easy lessons in locomotive running"
Published by International Correspondence Schools, Scranton PA.
The Westinghouse Type K Triple Valve volume.
It includes a fairly complicated little plastic schematic style model so you can move things around and see how various ports align as the triple does it's job.
If you are into that sort of thing and find a copy (with the models) score it, worth the price of admission and I *think* kinda rare.
Eventually I want to see the "Easy lessons in locomotive running" volume. Somehow I never envisioned "Locomotive running" as being "Easy".....
-Cp.