The fire-up yesterday (June 29, 2020) was to "blow out" the engine before installing the valves and pistons.
There has been a lot of work done inside the locomotive. Over the years, a lot of dirt and perhaps a few other things (rags, bolts, nuts, etc.) might have been accidentally left inside a steam passage. This is not a slam on the competency of the restoration crew...it just happens. They did this blow-out process after overhauls back in the steam days, too.
By opening the throttle with no valves or pistons installed, the steam has an open path from the boiler, through the throttle passages and out to the atmosphere. Allowing the steam to flow at high speed through all the throttle and steam passages blows out all of the foreign matter and confirms they are clean and ready for the installation of the valves and pistons.
Over on Trainorders, Wes Camp explained it a lot better than I did...
"When built, or heavily repaired or overhauled, so many different men had worked on the engine, and being human, they had “set stuff down”... sometimes in the big, open steam pipes. When assembling the pistons and the valves into their bushings, everything was nice and clean and wiped.
Then, at testing date, with the first strong chugs, “the forgotten stuff” ( tools, nuts, bolts, bars, etc.) got blown by the steam flow and lodged in the valve ports, or fell into the cylinders, often with disastrous results. Our grandfathers learned, the hard way, to blowout ALL the steam pipes BEFORE applying the spool valves. That’s the reason behind the steam-blow testing."
