The GWR and the LMS liked the Belpaire fireboxes, but the LNER and to a lesser extent the Southern went the other way. The GWR and LMS regions sent quite a few steamers to Dai Woodham’s scrapyard in Barry, from which many (most?) re-emerged to live again in preservation, which I presume is your experience. Virtually no LNER locos were so lucky, sold instead to more local scrappers who quickly cut them up.
CNR here in Canada had a few Belpaire boilered Mikados, but they were definitely the exception in Canada.
I understand that post-steam era boiler safety codes have been strengthened to enhance the factor of safety of locomotives used in the excursion era and that the PRR standard Belpaire boiler used on K4 and L1 class locomotives were found to be marginal even to the rules in place when these engines were in revenue service. But I think it’s a little laughable to think that the boilers used on these engines were insufficiently safe when properly maintained. That’s sort of what is being implied. The PRR had 425 K4 Pacifics and 574 L1 Mikados in service using this very boiler design, many for over three decades. If ever there was a case of a well-sorted design giving safe, dependable service, this was it.
I recognize the need to have robust regulations to maintain safety, and I think the updated boiler codes requiring a higher factor of safety than used during the steam era are a good idea. But the K4/L1 boiler’s service record speaks for itself. It’s hard to conclude there is a safety issue with that boiler design when one takes into consideration its service record. The new boiler codes merely add margin to what has proven to be a very safe design.
When I said ‘hopelessly incompetent’ I was intending only the LEGAL sense, first in the Boiler Safety Law context and then the evolution of Part 230. There is certainly something to be said for PRR building ‘light’ to save weight or adjust distribution and they were certainly not in the business of gratuitously endangering passengers or employees with defective boilers.
It can be surprising to note the actual thickness of metal needed to contain pressure. If I recall correctly the tractor at Mentor had plate wasting to inside 1/16" before it let go. So a little fudging with double-nutting is not really putting heads in jaws. However, do not expect either a historic ICC inspector or his modern-day counterpart to allow the non-compliant version to run…
Don, I forwarded your J3a story to Jack May, who enjoyed it immensly. I am about to post something regarding the difference between the K4s, 1954, and K4s, 1941, on the Classic Trains Forum, ref. pages 58 and 60, Winter 2019 issue.