I must admit to being a bit unsettled at seeing the photos of 487 with the scale buildup.
My initial reaction is that Chris is correct that boiler washes may need to be done more often.
I was on the engine crew of an excursion a long time ago. We worked the locomotive pretty hard and our water was bad. We got water from a reservoir rather than use city water because the city water was real nasty stuff. On more than one occasion I saw the water going into the tender and it looked good. We did start to have foaming problems which we handled operationally, including frequent blowdowns and blowing an entire bottle of water out of the glass every morning just to get fresh water. We assumed we were getting dissolved solids and the TDS was elevating. At that time the operator did not have a TDS meter. When we got home and opened the mudlegs, they had 8-12 inches of mud - solid. There was a large V-shaped canyon at the blowdown but everywhere else was packed solid. That was only 5 days. Turns out the tank truck driver was sucking off the bottom of the pond. Fortunately the sludge was soft and cleaned out easily. Additionally the grates are very high on that engine so the plugged sheets did not see the heat. Had the locomotive gone an entire month, even with perfectly clean water the rest of the time, the solids already there would have started to harden and distribute about the boiler.
Even when water does not look that bad, there doesn't have to be that much stuff suspended in the water to accumulate rapidly - particularly when a locomotive is used as hard as those on Cumbres. Unfortunately the test for suspended solids is not an easy one and is probably beyond most operations.
Hard water can be a problem. During a drought back in the late 70's, our water source turned hard. We built up enough scale in 60 days to crack a brand new flue sheet. Plugged a water bottle solid in less than a month. We finally found out what the municipal folks had done and we managed the situation. We use softeners now. We are lucky to need water at only one location.
The point of these tales is to illustrate that considerable buildup can happen in a hurry for no apparent reason.
A good water treatment program, in my opinion, uses the fewest chemicals possible. Additionally you should know exactly what chemicals you are using, and what effect, good or bad, they are having. I doubt that chemicals would help 487. Boiler washing, oxygen scavenger, and trying to maintain clean and reasonably soft water would be my major goals. Given the various water sources and the remoteness, I imagine that frequent, thorough, boiler washes would be the most practical. Oxygen scavenger is pretty straight forward.
I have often thought that water was one main reason we lost steam. Had they been able to lick that problem, there might have been incentive to further develop steam locomotion.
Water is one of the most neglected elements in steam tourist railroading, yet it is one of the most important. Bottom line is that it is expensive, time consuming, and sometimes puzzling, but you must know your water. Not knowing will lead to costly repairs at best, and structural failures at worst.