There you go then. Great business opportunity for such a loco! You just need a catchy name , an annual upfront fee per rescue loco, few standby drivers. Could be a money spinner for rail preservation trusts.
PS who runs and owns locos that pull freight trains.................?
The problem, or fact is, that modern classes of EMU are designed to work in sets and their electronic brains are now very difficult to connect to by a traditional, now outgoing, diesel locomotives. It is hard to even slip an addition coach into a 3 or 4 car set, and that is why you regularly see two, or even three, EMU complete sets joined together drivers power car to drivers power car.
There couplings and general connections, like air brakes, can be different even between EMU types, and totally unsuitable for locomotive connection, being often at different heights to each other.
As LCO112G correctly stated spare diesels are now not an option, with even the freight operators examples not compatible with most of the EMU's. In addition, as stated, those freight operators have nothing directly to do with either the passenger train operators or Netwoork Rail, so are not expected, or can afford, to hire their loco's out, and that would be very expensive. Each loco journey costs a large fee to Network Rail, and for them to drop their freight duties to assist another rail company and accumulate huge mileage costs as they travel to help a stranded EMU, is just not viable from a commercial point of view, or desirable for profit hungry freight companies and their passenger company opposite firms.
As for steam locomotives being used as muted not that long ago that is a non-starter. There are not enough of them, let alone those air brake equipped; there are all the H&S implications; the maintenance costs, would be enormous, if they are available in the part of the country where they are required, and there is still that compatibility issue with the EMU's!!! There would also be the little issue of the Environment!
So the suggestion was either an April fools joke, or some kind of stupid suggestion by a journalist who knows nothing about railway operations.
https://www.theguardian.com/commentisfree/2018/feb/12/diesel-steam-trains-rail-electricity No, the answer is to build and maintain the latest EMU's to a far higher standard so the chances of them breaking down is relagated to "rare".