The British Railways Sulzer Type 2 – classes 24 and 25
This is an introduction to a class of locomotive that could at one time be found just about anywhere on the British Railways network, handling both freight and passenger traffic - hence their sobriquet 'Rats'. They were on the drawing board way back in 1955 and were among the first generation of reliable diesel locomotives that paved the way for wholesale modernisation of the Britain's rail transport system. The Type 2's were later classified as Class 24 and Class 25 and members of both classes lasted until the 1980s A few made their way into preservation and during 2010 both classes could be seen on the Gloucestershire Warwickshire Railway: resident class 24 no. 24081 and visiting class 25 no. D7612 (25262).
The British Railways modernisation plan, launched in 1955 at a time when the Standard steam locomotives classes were still in production, envisaged a series of diesel locomotives that would eventually take over from steam. One of those on the drawing board was the Sulzer Type 2 Bo-Bo diesel-electric locomotive, series the first of which was delivered in 1958 and allocated number D5000.
Production of these compact locomotives continued until 1961 and eventually 151 were delivered. They were built at Crewe, Derby and Darlington (in fact, this was the first diesel locomotive class to be built at Darlington) and, unlike the Standard steam locomotives built at around the same time, were quite long lived.
Although most of the class initially worked around the Derby and Crewe areas handling both passenger and freight (several were fitted with train steam heating boilers), they could be found pretty much throughout the UK. Several worked on the Southern for a time, some in and around London including the Metropolitan lines, in Wales (particularly the former Cambrian route) and Scotland. They became associated with the 'Condor' fast freights between London and Glasgow until introduction of the Freightliner services in 1965. Some were equipped with air compressors and allocated to Gateshead depot to handle the Consett iron ore trains, the bogie wagons of which had air-operated hopper doors. They worked in tandem on these services, taking over in 1966 from the air-pump equipped 9F class 2-10-0s which previously handled this traffic. Further members of the class were based at Inverness, working passenger services in the Scottish highlands.
The power unit was a six-cylinder turbo-charged Sulzer 1,160hp diesel driving through 735Kw British Thomson-Huston (BTH) electric transmission to traction motors on each axle. Top speed was 75mph.
The last of the class 24s to be withdrawn was no. 24081 (D5081), ending its career at Crewe in 1980. This locomotive is now privately owned and is in working order, finished in BR blue and based on the Gloucestershire Warwickshire Railway. Just four of the class survive.
The Class 25 is a development of the class 24, continuing where the class 24s left off in 1961. They were similar in most respects to the class 24 but the Sulzer power unit was improved to produce an extra 90hp, to 1,250hp with the addition of an intercooler. This gave the locomotives a rather more useful top speed of 90mph.
Production of the class was shared between the BR works that produced the 24: Crewe, Darlington and Derby plus one outside contractor, Beyer-Peacock. In all, 327 were completed, production continuing until 1967. They were primarily intended for freight traffic although several were equipped with passenger train steam-heat boilers. They could be found pretty much throughout the British Railways network during the 1970s although none were permanently allocated to the Southern or Eastern regions. This widespread distribution earned them the nickname 'rats' - because they could turn up anywhere!
There were a number of sub-classes with variations to the power train, control systems and brakes and minor styling changes.
The number series was:
class 25/0, D5151-D5175, (25001-25025);
class 25/1, D5176-D5232, (25026-25082);
class 25/2, D5233-D5299, D7500-D7597 (25083-25247);
class 25/3, D7598-D7677 (25248-25327).
Some of the 25/3 series were designated 25/9 for mineral traffic that in fact never materialised.
Last of the class were withdrawn in 1987.
D7612 (25262), which was a star visitor to the Gloucestershire Warwickshire Railway for the Summer Diesel Gala in 2010 and remained for several weeks, is a member of sub-class 25/3, built at Derby in 1966. This batch was equipped with electronic speed control, dual air and vacuum braking and also originally had a train steam-heat boiler. It is smartly finished in British Railways two-tone green livery.
Twenty of the class survive, D7612 being owned and operated by the South Devon Railway Diesel Group.