I suspect that we may see yet see coal burning steam engines with electric transmission, and probably burning pulverised coal, as suggested in previous posts.
Electric transmission has many advantages as detailed in the earlier post, to which may be added, the relatively easy generation of power for ETS, which is a problem with traditional steam.
Steam turbines, alternators, and electric traction equipment are standard off the shelf items.
Pulverised coal burning has its merits, but also two significant drawbacks.
Firstly, if the loco is to be filled with lump coal to be pulverised on board as needed, then that adds expense, weight, complication, and another point of failure. If instead the loco is to be loaded with ready pulverised coal, probably in a converted tank wagon, then that adds certain risks.
Fine coal dust is explosive and requires care and precautions in handling.
The other drawback is that pulverised coal burning equipment is not so easily started and stopped. If the steam supply is excessive, and fireing therefore ceased, then starting can be innvolved.
One option that was considered in the USA some years ago, was to equip the fire box with a small grate for lump coal, this grate being sized so as to keep up steam pressure when the engine is stopped. Pulverised coal then to be blown in under air pressure as needed, it would ignite instantly and reliably due to the small but hot pre-existing fire in the grate.
Hand stoking this small grate would be not much more work than a domestic fireplace, the great majority of the heat input being from pulverised coal.
There was considerable interest in this technology in the USA at the time of the Arab oil embargo in the 1970s.