Kelly and Mr. Knoob - thank you for the clarifications.

Very interesting. I think it is unlikely the piston seized since she was apparently well broken in (i.e. showing signs of a lot of in service wear) so the piston wouldn't fit the bore so tightly as to bind without some large amount of foreign material gumming it up, which wouldn't be a sudden sort of thing. The rings would compress into their grooves when overheated providing even more room to freely vibrate while, as Earl says, the Johnson Bar would be doing a frantic dance. Others comment that the bore appears to be in unmangled condition - very good. It seems likely that the piston itself may have knocked a chunk of itself loose due to some hidden flaw combined with the intensity of working the heavy grades which got propelled towards the front, caught between the rod end or piston and head, and mayhem ensued. Since the engineer was quick on the draw about shutting off at the first sign of trouble he's experienced and savvy enough to have also stopped if the Johnson Bar had started thrashing him near the crotch, so the oil scenario doesn't (forgive the pun) seem to hold water.

[quote="Kelly AndersonPRR #1223 never had bump marks on her left crosshead guide showing where the piston hits the head, so the first chance I had when the main rod was disconnected, I put some on. She has a 26" stroke, and the bump marks were 25-15/16" apart! quote]

The Standard Railroad of the World. Nice catch.

Neither - the Hermosa water tank was dry, so they siphoned water from Hermosa creek.

Given the current technology, would using centrifugal cast or ductile iron be the quickest way to make the replacement piston and head?

Video of the event.

http://trn.trains.com/news/news-wire/2019/04/video-southern-pacific-no-18-blows-a-cylinder-while-on-dsng-excursion

It is well known that in vertically oriented pistons on marine applications running saturated steam no lubrication is supplied to the valve or piston. Superheat requires lubrication and intense oil removing procedures for condensate destined for boiler feed water.

Since speculation is rampant, I'd guess at stress/fatigue cracking due either to an internal flaw in the casting that finally revealed itself, or the design of the piston. It'd be nice to know if the engineer had any warning - he did shut it down fast. If it broke without preceding symptoms, then I'd lean towards fatigue. But what do I know about steam... I'm a lowly diesel guy.

True but then in a vertical motion the condensing water would evenly spread over the ring to lubricate it. With horizontal the water would not reach the top as easily.

Robby Peartree

What would the advantage be to getting new parts cast, vs. just getting them CNC'ed and milled?

If there's an advantage it isn't going to be production by removing everything from a billet that doesn't look like a piston or casting something limited to what does, but in doing a better piston than Baldwin put in it - multiple rings in a better designed spider or a built - up variety - there's a lot to be learned from what happened after the 1920s in locomotive technology. If I were convinced it was a lubrication failure, I'd also want to add supplemental force fed mechanical lubrication too..... just sayin'.

Another video turned up, and what may be of interest here are the preparations made to take the crippled 18 home, starting around 45:00:

https://www.youtube.com/watch?v=tVh0lpmgJ3Q