Much of what all of you fellas have written is for the most part accurate.
There were however, some few builders/railroadss that tried to do the "modern thing" with their steam locomotives.
As for mass produced locomotives with interchangeable parts, UP requested that their 4-8-4s, 4-6-6-4, and even the 4-8-8-4 had many parts that could be utilized on the differing engines. Then there was the
advances made by N&W with their J class, A class, and Y6bs. Which were late built steam locomotives. Particularly the Js, which had very modern, for the times, mechanisms and appliances.
Had the modern," thirtys built steam locomotives" not been beat to crap during the Second World War, out of necessity, and the advances not been made in the improvements in the diesel prime mover up to and includeing the 1939 FT freight set, I feel there would have been some kind of a modern, steam locomotive.
Due to critical need during the war years, the internal cumbustion engine and metalurgy used in same made giant steps forward. These improvemnts sifted over to the diesel engine also. And only in a few instances did this same metalurgy and sience transfer to the steam locomotive.
If one takes a look at the history of machinery in the last fifty or so years, its clear that the steam locomotive was at its zenith for the times, and the diesel was being developed into the dependable machine we all know. That developement energy was not extended to the steamer.
I only wish I knew that if the same or even more modern technology was incorporated into the steam locomotive, wheather or not we would have some sort of modern steamer in regular service today.
Take care,
Bruce