I seem to remember something similar happened with the 50s and 55s in their twilight era, and would anybody use them to operate higher-speed services?
One class 50, renumbered 50149, was fitted with regeared class 37 bogies to assess the class' potential for working Cornish china clay trains, and one further loco, 50015, passed into the civil engineers' division at the very end of the class' life, but neither venture proved a success. As for the Deltics, as far as I am aware they were retained on East Coast passenger workings until the end of their lives in regular revenue earning service: Even at quite a late stage in 1981, the locos were being considered for a second lease of life hauling North Transpennine trains starting from the following year.
In a similar fashion, and as has been set out in the posts above, the HST power cars are very specialised vehicles that are not well suited to hauling freight trains.
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The Thrumpers were passenger trains and became excellant freight haulers in Norther Ireland. HSTs must be able to haul wagons if re-geared.
The NIR 70 class proved themselves quite adept at hauling (in pairs) a daily Londonderry to Lisbane freight train, but they weren't necessarily best suited to such work, even with a comparatively low maximum speed of 70mph.
Most of the first generation DMUs introduced to Britain in the late fifties were capable of handling a modest "tail load" of wagons, but they weren't considered well designed to handle such loads.
