The Class 67 locomotives were designed (as many have been before) as "mixed use"- in this case, optimised for fast mail and passenger services.
Chiltern hire in a number of them to power trains. These have been fitted with remote fire fighting equipment required to allow them to be operated from the Driving Van Trailer at the other end of the train without someone sitting in the loco keeping an eye on it. Usually this will be the ones in their Silver (ex WSMR) livery (67012-15), but if those need maintenance there are a few more in the DBS fleet that are fitted- I know that 67018, in the DBS Red livery, is one these.