101 of the class were built between 1961 and 1964, when it became apparent that there was a requirement for a medium-power diesel-hydraulic design for both secondary passenger work and freight duties.

Once they had proved themselves more than capable of handling these duties, they were also assigned to express Paddington-Cardiff-Swansea services, displacing King-class steam locomotives.

These duties were heavier than they were designed for, and the Hymeks were displaced when Western and Brush type 4 locomotives became available to allow accelerated timings.

Hymeks also worked pickup freights throughout the Western Region as a mixed-traffic design and were used heavily on inter-regional passenger services.

Hymeks were used all over the Western Region on mixed traffic services from secondary passenger and parcels through express freight to ballast trains.

The simplest way to avoid excessive wear, and the stopping of a train on the incline, was to lock first gear out of action, via the master switch located in each locomotive's A-end cab.

In the early 1960s, yellow warning panels were added to the lower part of the front ends, in accordance with BR's then-new policy.

With the advent of the Corporate Identity scheme in 1965[13] some locomotives received all-over BR Rail Blue with small yellow warning panels.

The National Traction Plan of 1967/8 decreed that designs proving unreliable, expensive to maintain or non-standard should be eliminated as quickly as possible in order to reduce the number of diesel classes from 28 to 15 by the year 1974.

The engineering factions of the British Railways Board, the body that oversaw BR's operations from 1962 onwards, felt that all of the Western Region's diesel-hydraulic fleet should be counted as non-standard and should be withdrawn as quickly as possible.

They were replaced by Class 37 diesel-electric locomotives made redundant in other regions as a result of a general decline in railborne freight traffic throughout the 1960s.