I attended the C&TS Fireman School earlier this month. To a green fireman, it sometimes felt like threading a needle to keep the pressure up without popping the safety valve. I hated when the safety valve would pop because #1, it was annoyingly loud and #2, it was a painfully obvious waste of water, fuel, and effort.
Too little properly-placed coal or too much water added too fast relative to steam consumption and the pressure would drop. Too much coal, not enough steam usage, and not enough water added and the pressure would climb. If the pressure was climbing and the boiler couldn't or shouldn't take more water, the safety would pop.
If I recall correctly, the engineer wanted to see #190 of steam pressure when we left Chama, and the safety would pop at #195. He wanted at least #175 to keep the air compressor happy when we had no other need for steam. My goal was to keep the pressure as close as possible to #190 when needed without going over. It became a game that I wasn't very good at, especially at first. If we weren't using much steam, I'd start to get a little nervous north of #185. By #188 or so, I was reaching for the injector to get it started before we crossed about #192. There is an art to fining down the injector to put water into the boiler more slowly and it took me awhile to start getting the hang of it. Not knowing the line like the back of my hand also made it difficult to properly anticipate the need for steam. I think that with more training and experience I could be a decent fireman, but it was a bit overwhelming at first. There are a lot of factors going on in addition to watching the left side of the track. I plan on taking the fireman school again next year. There is far more to appreciate than I could comprehend in a few days, and I know I can do better.
As a side note, I rode the D&S a few days after leaving the C&TS. I have to give credit to the fireman on #486 that day. He kept the pressure up and popped the safety valve many times on the way up to Silverton without unnecessarily dark smoke. I assume that he kept the boiler was full and could not add more water to cool it off. Although it was not a particularly efficient way to run, I have to give him credit for maintaining an efficient fire. Having stood in fireman's shoes, to have the endurance to keep scooping that much coal is impressive. To scoop coal and place it where it is most needed and keep the stack relatively light at that rate also indicates some pretty good skills. For the same effort, that fireman could have fired a locomotive in a situation calling for more steam.
Dave