schlimm:
Not really sure, but I think I read that Diesel fuel back in the 50's was cheaper than gasoline, so maybe it would have been relatively cheaper in your calculations than now.
The story back in the "Steam-Diesel Transition Era" was that oil was being discovered/developed in Saudi and the US hadn't reached it's 1970 Hubbert Peak, and the United Mine Workers were flexing their muscles with regard to the mine workers getting better pay for the dirty dangerous work they do. Oil was particularly cheap compared to now, and it seems with the low price of oil and the much higher thermal efficiency of Diesel engines, Diesel locomotives had a clear fuel cost advantage over steam.
At the time of the ACE 3000 project -- what was that, mid '70's in response to the 70's Oil Embargo and Middle East tensions -- I believe that a mid teens thermal efficient coal burning steam locomotive had a clear fuel cost advantage over a Diesel locomotive. Even a 5 percent efficient steam engine (the ACE people tested a Northern) had a slight fuel cost savings over Diesel at that time.
Today, the price of coal is increasing along with many other things, but the price of oil in inflation-adjusted terms is not too far off from conditions in the 1970's -- early '80's.
The other thing to remember about the ACE 3000 project is that when was this, early '70's, and when did Norfolk and Western drop the fire of the last mainline steamer, 1960? Yes the railroads were thoroughly Dieselized and the thought of bringing coal-fired steam back -- think of Don Oltmann's shop foreman thinking the best thing to do with a locomotive boiler was to fill it with cement so it could not be put in service and have a boiler explosion accident. But there were probably enough old-timers around the railroads to make this steam thing work, if that was the direction.
Today, we are 40 years away from the ACE 3000, 50 years since the end of steam on N&W, 60 years since when the railroads pretty much pulled the plug on steam. During the steam-Diesel transition, the railroads were "investing" in a hodge podge of 1st Generation Diesels with various levels of maintenance expense. By the ACE 3000, they were competing with the SD-40 and later SD-40-2, perhaps the most "bullet proof" locomotives known to mankind, steam or Diesel.
Ah steam! The dream doesn't die.