This secondary route in Switzerland is threatened with closure, so the suggestion has been to run it with steam traction. Out came the rebuilt Kriegslok, rebuilt by DLM but originally constructed to run for just six months during the war, and duly ran the service on a trial basis last autumn. The embarrassing thing is that it is an excellent performer, probably the equivalent of a class 66, with an axle load of 15 tons, even less than an HST power car. It runs on light oil and has a very clean exhaust, with no black smoke, easily meeting current emission standards. Experience with the Brietz-Rothorn locomotives has been that modern steam engines consume less fuel and are cleaner than the diesels.
Whether this results in an order remains to be seen, but it could set a precedent. We shall see. You can't electrify everything and there are obvious niche applications everywhere for a small fleet of such locomotives for intermittent duties as infrastructure trains, rolling stock transfer, rescue and charter services. If that worked, it might be a way of dealing with the problem that the Inter City Express do-everything train was meant to solve, but at a fraction of a cost, as well as being an inexpensive fix for keeping in service British Rail's second generation class 15X DMU fleet dating from the 1980s.
Elsewhere, there are many railways that will never be busy enough to be worth electrifying, and others which were electrified long ago, where the overhead wires will not be worth replacing when they wear out. External combustion locomotives also make it possible to burn any fuel that happens to be available, including waste biomass, which would make them better than carbon-neutral.
The idea of using kettles sounds bizarre but worth keeping an open mind about.