Quote: "It's not obvious until you look at the drawings how little water is on top of a crown sheet."
I've noticed that and have always wondered why there hasn't been more failures considering their widespread use in traction engines. Looking at the design and considering an engine going down a steep grade for a considerable amount of time seems that it would have resulted in the loss of the fusible plug at least. Coupled with fired boiler under very little load going down hill, would have ran the pressure higher, all adds up to high probability of a mishap.
The more I learn about steam, that greater distance I keep myself away from traction engines.
I was at one antique tractor show and they were having a parade, and an old not-too-well-maintained looking Case engine waiting in the line popped off and shot steam 50 feet in the air for a good five minutes, while the operator was beating on the relief valve trying to get it to reseat. If the relief valve is like that, what else is and may be even worse.
A good consideration regarding the possible state of some of those antique boilers, steel applied in similar duty more less deteriorates at the same rate over time, the Medina, Ohio engine had deteriorated to total failure almost 20 years ago.
-Ron