Many of the new diesel locomotives that British Railways ordered to replace steam were either useful for tasks that were no longer needed or simply useless, yet the solution to the need for a flexible, adaptable, light freight locomotive already existed.

While such white elephants as Class 17 (1962-65) and Class 14 (1964-65) were devised, ordered, constructed and found wanting, the first batch of 128 English Electric Type 1 locomotives, later designated Class 20, had been built between 1957 and 1962.

They conformed to the misguided thinking of the time, ironically – a design based on the American switcher, rated at 1,000hp with a maximum speed of 75mph, lacking a train-heating boiler and so unsuitable for passenger trains except in hot weather.

The cab filled one end, and much of the locomotive frame was given to a long bonnet which concealed the single English Electric power unit, its weight providing adhesion for hauling heavy loads.

By the mid-1960s British Rail were lumbered with various patterns of unsuitable locomotives as the need for light freight locomotives declined, and the Class 20 proved adaptable to a range of purposes.

Crucially, they were capable of operating as multiple units – two or three locos driven from one cab – so they could handle heavier loads without increasing crew costs.

Marshalling them in pairs nose to nose provided the driver with maximum visibility of the road ahead in either direction.

Some even appeared on passenger trains, in tandem with other classes fitted with heating boilers.

Indeed, in 1966 BR ordered another hundred Class 20s to replace the ragbag designs that had to be junked quickly.

They lasted well, and turned up in unexpected places, such as the construction phases of the Channel Tunnel and High Speed One.  Of the original 228, 39 are still in existence.

They have proved popular with the leasing companies that supplement current operators’ traction needs, such as Direct Rail Services and Harry Needle Railroad Company (HNRC).

And they are usefully employed on heritage railways. Known to enthusiasts as “Choppers” because their exhaust resembles a helicopter, their noise is instantly recognisable, redolent of a particular period of British railway history, shortly after the demise of steam.