25258 near Walsall in September 1978. © Steve Jones
The Class 25 (aka the Sulzer Type 2) were a class of diesel locomotives produced for British Rail. The initial batch of Class 24 locomotives were deemed to be lacking in top speed, so the Class 25s were given uprated engines and charge air cooling which allowed them to beat the 75mph top speed of the 24s. Although primarily designed for freight work, a number of the class were fitted with boilers for heating passenger trains. Throughout the 1970s the class could be seen across much of the network (aside from the Eastern & Southern regions), which led to them being nicknamed "Rats". There were a number of variations present amongst class members, with the initial 25/0 type followed on by the 25/1 (which featured different traction motors and other modifications), 25/2 (which featured restyled bodywork and a different initial livery) and the 25/3 (the final batch of locomotives). Some of the class were converted into mobile generators for train heating in the 1980s, which were in turn scrapped by 1994.
(Information provided via Wikipedia)