Re: RGS #42

Assumptions mostly. Baldwin's engineering department was always tweaking the performance and metallurgy of their locomotives. In six years they had to come up with new and different ways of doing things and this was certainly apparent when you compare the layout and equipment of a C-17 vs a C-16 or C-19. We know they had a rolled steel boiler instead of a cast iron one. As John Coker points out below, this eventually led to their early replacement because they had lap joint boilers and didn't get new ones in the teens. They had a totally different springing arrangement with these 2-8-0s than what they were doing six years earlier, with the mains hooked to the third driver and improved aligator crosshead guides, Stephensen valve gear moved to driver three instead of driver two with resulting longer links - some sort of more efficient arrangement perhaps.
Just working on the knowledge that every new design from Baldwin had "improvements" over the earlier design. Some worked, some didn't. It was undoubtedly the age of the boilers and their method of construction as well as the lack of need for smaller power that caused their demise. I'm just glad the Southern got the 420/42 and she was able to survive for us to consider and debate.