Another comparison from the Milwaukee Road.
To pull an excerpt from the above article:
"Railfans, and often railroaders of that era who weren't actually looking at the cost numbers, mistook the substantial decline in shop forces of that era as having some "special meaning" regarding the maintenance requirements of a modern Steam engine.
The decline in shop forces mirrored three significant post-War influences: 1) automation, 2) the decline in tonnage hauled during the period of Dieselization (a decline which averaged about 42% on US railroads, which just about matched the percentage decline in shop forces), and 3)the wholesale replacement of a motive power type with an average age of 27 years (on the Milwaukee), and near the top end of their service life maintenance curves, with machines that were at the bottom of their service life maintenance cost curves."
The article goes on to talk about economic service life of steam vs diesel, how GM reduced the ESL from the touted 20 years to 14 years in 1954. a 43% reduction, etc.
From what I understand, for the roads that had standardized, modern superpower and a competent maintenance program, the only reason the change over occurred was the otherwise wholesale abandonment of steam power, due to low interest rates on new diesels and the havoc war time traffic played on an generally aging steam fleet, forcing the closure of companies mass producing parts like feedwater heaters, rods, etc.
One only has to look at the operating ratio of the N&W and NKP vs. their dieselized competition in the last years of steam to see this. (In 1951 the NKP OR was 68.2% vs 80.2% average for all Great Lakes Region class 1s. In 1954 the N&W had an OR of 66.4%, with its grades and passenger service, it was still in the nation's top 5. Norfolk Southerns OR stands at 61% today, it's best ever, in the midst of PSR, for reference)
Or in the case of the NYC, a president who wanted to keep up with the Joneses and hated the image of steam, despite the research of the motive power dept. See Paul Keifers (Chief of Motive Power, NYC) book, published in 1948 titled "A Practical Evaluation of Railroad Motive Power"
Come to think of it I believe I read somewhere that the N&W saw a leadership change that effectively enforced the same mandate.