I've often wondered why somebody hasn't built a little magnetic robotic camera that would crawl the interior (or exterior, i suppose) of a boiler doing the inspection remotely. If an autonomous rover can grind rocks and do spectrographic analysis on Mars for a year or more, it seems like building one wired robot with electro magnets for feet and a fifty foot long tether wouldn't be that expensive.
After all, it wouldn't even have to pass any crazy NASA regulations, and if it did break down, you could cut the power and use the wire to drag it back out the steam dome.
I presume the inspections themselves are all just eyeball and occasional hammering and scraping? If so, that would be even easier than if it had to take samples for analysis.
Perhaps there's some smarty pants electrical engineer at Los Alamos or a western University who would really like to learn to operate a unique historical locomotive in exchange for a baby robot?
Tom Stewart