Do not forget the Federal requirement for Steam Locomotives. After 1,472 days of operation, the locomotive must be torn down for the FRA Inspection. Check the boiller and tubing, fire box and stay bolts, safety valves and injectors, etc. And who does it? As I remember, after the Gettysburg boiler explosion, the government had to bring in “experts” from the Strasburg Railroad and Essex’s Valley Railroad to show them how things should be done in today’s limited steam use.
That’s a lot of labor. Also, unless a LARGE number of Steam Locomotives were built, the cost of replacement parts would be “off the wall”, much of the time hand made.