I would like to put my two sence worth in on what a modern steam locomotive could be to use today's technoglogy and compete with diesels.
First, I would use electric transmission, with ac motors, an alternators for motive power, like modern diesels, possibly using iderntacle equipment.
I would use modern power-plant technology, possibly natural gas fired, since that seems to be the future low pollution nearly infinite resource fuel available in North America.
There is a somewhat new development in power plant turbines. Use of partial circumference power. Imagine that at each quarter point around the circumference there is an exhaust port immediately followed by the injection port. The turbine can operate at full speed with only the two opposite quarters working, two on each side or top and bottom (keeping rotational balance) for half power or with all quarters working for full power at the same speed. From what I have read, efficiency can be high in both modes.
So here is a 3750HP locomotive, with two turbines, one 3000, one 750HP, each also efficient at half power, same RPM, each with its own AC alternater. The smart electroncis for control, rectification, combination, and frequency control for driving the motors. Trottle positions:
0, idle, Run 1 375HP, 2 750. 3 1500. 4 1875 5 2250, 6 3000, 7 3375 8 3750
Sort of a mineature power plant on wheels, with high pressure boiler. and possibly, as others suggested, the reuse of old diesel frames and trucks.