If it has been pointed out that the average of the last year of use of any steam is a misleading metric, but you don't agree, OK. But to imply we are just spouting philosophy is wrong . If I could find the metrics that would shed more light on this I would. You have thrown out numbers, but you fail to source them so that others could share
Oh my goodness. I looked them up. Google. Mostly from the railroad historical societies, and a couple that I remember from when it happened.
Maybe that's my point. Simply gainsaying some published numbers by essentially claiming, "no that's not it yet, keep trying," isn't much of a discussion.
Other than having personally witnessed the transitions on two different railroads, I don't think I can be much help at what you are looking for. My point with Illinois Central is that it wasn't really a late mover compared to several other similar Class I railroads and I doubt there sufficient value in the story of its conversion to diesel to support or disprove the thread premise either way.
And N&W wasn't that late to the game. It was a wealthy road with lots of good coal. There is a basis to the premise. But then Northern Pacific was a marginal road with lots of crappy coal. It dieselized the same year as N&W, as did several other roads without a particularized historical attachment to coal, nor a definable consistency of economic health.
So, how is the "metric" misleading? I can't tell from the comments.
I will say this from some experience. The carrying costs of a steam operation that is being replaced by a diesel-electric operation are high. Once a railroad management made the commitment, it was generally done as quickly as possible. GMAC made this possible by extremely good financing terms.
A railroad would not want to keep the last 20% of its steam fleet on the property for more than a few months of operation. The unit costs, and the unit/mile costs, start to go through the roof. A management that is awake is not going to keep that last steam engine limping around, and all of its support facilities, and labor, for more than a few weeks. The "Mohawk that Refused to Abdicate" kind of stories refer to a matter of days, not even weeks, when whole divisions were converted to diesel.
At the same time, and this was "fortuitous" in a twisted sense, tonnage on Class I railroads was dropping and so the whole process of dieselization not only sped up considerably in the time period 1956-1960, but the estimated fleet needs declined considerably. All of a sudden, boom, steam disappeared more rapidly than expected and railroads found themselves "dieselized" in many instances well ahead of schedule. And that wasn't because of quickened delivery of diesels, it was because of declining traffic.
As a "fer instance," I was visiting with a shop foreman, and the shop had just finished an overhaul of an S-2; completely rebuilt, new glossy black paint, white trim, a beautiful, graceful, powerful, modern, efficient steam engine. He had gotten a phone call to cut it up. Some of the men were almost crying. Some of them had kept her and her sisters going their entire careers. That was it. Reduced to scrap. Gone. The end "of Steam" was that sudden and unexpected.
Perhaps it would have been useful to "be there". The opinions offered might be different, I don't know.
I've got some fleet numbers around somewhere that shows motive power TE for successive years in the 1950, I will try and dig that up and see how fast the total numbers declined. But, these "facts" are available in the Transport Statistics of the United States and are not just unavailable to the point that speculation and gainsaying need to take the place of looking them up. I am reluctant myself to start taking more trips to the library because my recent experience in that effort wasn't particularly useful as against ingrained belief systems, although it was an illuminating experience for me, as I have always found facts to be.