lets say N&W 4-8-0's, you had hundreds of them, rather than swapping boilers, you put another engine in service while that one is being shopped.

I don't know if the N&W did any boiler swapping, but I know they did a lot of boiler work on existing equipment, and the 4-8-0's are a testament to that serving nearly 50 years in service. The need to swap boilers may not have been as necessary as the ability to drag another engine out.

I don't know the ruggedness of Britains boilers vs US boilers, we know about wear and tear, but for a boiler to last the near 50 years like the 4-8-0's say....RUGGED.

And we have some steamers nearing 100 years of active operating on tourist lines..keeping in shape, I'd say boilers have lasted very well here.