It's as much a "feel and art" thing, but it's more like a Flow chart:
If this---then that
If not this---then do that
You have to monitor more than to dozen operations at the same time, and listen, smell and feel how the locomotive operates, it's as complex a machine as you can get, and the space shuttle guys have it EASY because they have a computer to monitor most processes, and warning lights and other gizmos and widgets for safety. If you're lucky you'll have a low-water alarm in some of the most modern steam locomotives, otherwise, you have to constantly check sight glasses, adjust valves and such.