I think one can always find exceptions to the norm in regard to operation of locomotives from 1960 to the end of steam. I am thinking more of what locomotives were normally assigned to do. The MAIN purpose of the King locomotives, which were assigned to Laira, Old Oak and Wolverhampton was to operate the Paddington expresses. My point is that before the rundown of steam locomotives it was not usual to see them on workings they were never designed for. Shedmasters towards the end of steam must have had a hard time finding serviceable locomotives as BR reduced maintenance and many locomotives were just withdrawn on shed due to unserviceability. There is no end of pictures of express passenger locomotives operating freight trains for instance post 1960.
The most interesting example, I think, of locomotives operating services they were never designed for is the Pines Express which were very successfully operated by Standard 9Fs, built for heavy freight trains, on the old Somerset & Dorset line to Bournemouth