The Class 31’s were among the earliest diesel locomotives on the UK network, making their debut in 1957. A small body size and lightweight design however made these engines incredibly popular, and even today they still see work on the mainline network.
Construction of the first locomotive was completed in the final week of September 1957, and the handing-over took place on 31 October. The Class 31 entered service in November 1957, after the launch of the Class 20 locomotive and was one of the Pilot Scheme locomotives ordered by British Railways to replace steam traction.
They were originally built with Mirrlees JVS12T 1,250hp and 1,365hp engines and Brush electrical equipment, but the engines were not successful and in 1964 D5677 was fitted with an English Electric 12SVT engine (as used in the Class 37) de-rated to 1,470hp. The trial proved successful, and between 1965 and 1969 the entire class was re-engined. The de-rated engine was used as it was the maximum the electrical system could accept.
In all, 263 of these locomotives were built when production ended in 1962, and their influence was felt right across the UK network. The flexible and reliable nature of these plucky machines made them ideal for nearly all walks of life, from engineering trains to passenger services. On passenger services they were particularly ideal, especially those which had to make frequent stops.
However, by the 1980’s and 1990’s the introduction of DMU’s forced these locomotives off their passenger operations, and with the introduction of Class 60’s and Class 66’s for mainstream operators such as EWS brought an end to their mainline freight services. By 2003 barely anything remained of their mainline operations aside from departmental
Today, the only remaining Class 31’s on the mainline work for Network Rail, hauling test and measurement trains up and down the country. As of 2016, four locomotives were owned by NR, two of which are currently for sale. Aside from those on the mainline, 26 of these engines have been preserved across the UK, including the first and last locomotives built.