Regarding the practicality of US large steam, I read that when the Union Pacific 4-6-6-4 Challenger was restored and running just odd excursions in the US west, the desirable coal was in Pennsylvania. Some five hopper cars were in exclusive service running to and fro with coal, just to keep one loco and these periodic outings going. Eventually the loco was converted to oil firing, the residual (and cheap) Bunker C low-grade fuel oil still being readily available in the western USA.
The recently rebuilt/restored UP 'Big Boy' 4-8-8-4 has also been converted to oil firing (they were all originally coal fired). Many of the Challengers were converted from coal to oil firing after WW2 while in normal service.
And of course, much of the Southern Pacific steam loco fleet was oil burning (for the fuel cost/availability reasons you mention) - which allowed the development of the
SP 'Cab-Forward' articulated locos by Baldwin (rotate loco through 180 degrees, attach tender to smokebox end of loco and pipe fuel oil and water through to firebox/cab).
I think I've mentioned before that some of the later US steamers just about equalled overall costs of the equivalent amount of diesel power at the time. I can't remember where I saw the figures but they're fairly readily available I think.
AFAIK, the Norfolk & Western railroad went on record (in the late 1940s/early 50's I think) to say their modern big steam power was as economical as diesel power.
But remember they made their money hauling high quality coal out of the mountains to the Atlantic coast (so their fuel costs were low as it was on the doorstep, and burning coal was good for customer relations), they designed and built the locos themselves to suit the operations, stayed with compounding for maximum fuel economy, installed coaling bunkers over the mainline tracks (so locos didn't have to come off trains to refuel) and streamlined the day-to-day servicing operations as much as possible etc.
In other words if any US railroad could keep big, modern, steam competitive with diesels, it was probably the N&W. But even they were fully dieselised by 1962, so what does that say about the relative economics of steam versus diesel?
*EMD* did well - Alco, Baldwin etc, not so much ( Alco at least improved a bit eventually ) - which leads me to wonder if any notable steam builder anywhere made a really good job of diesels.
Alco did OK - they invented the 'road switcher' loco design well before EMD embraced it, their diesel engines were generally OK (with the exception of the 244 series) and the GE electrical equipment was good - Alco's were regarded as better 'luggers' than EMD products. But they seemed to have too much historical baggage and never got their production costs as low as EMD, so in the end just pulled out of the US loco market and concentrated on other, more profitable, things instead (their affiliates and licensees in other countries carried on producing the Alco designs for some considerable time afterwards though).
It was difficult for a major steam loco manufacturer like Baldwin (the largest US steam builder) to change to diesels - your main market was going into sharp decline while at the same time you were having to develop and market a new, unfamiliar, product range to replace it. Plus you need a lot fewer diesel locos than steam to haul the same amount of traffic, so your huge production facilities and headcount were going to have to be downsized to match your likely production needs. Alco did a lot better because they were much more serious about diesels earlier on, and had a partnership with GE for a good while (until GE decided to go it alone and market their own mainline diesels in competition, which was probably the beginning of the end for Alco, really).