The British Rail Class 45 or Sulzer Type 4 are a type of diesel locomotives built by British Railways' Derby and Crewe Works between 1960 and 1962. Along with the similar Class 44 and 46 locomotives, they became known as Peaks.
The engine of the Class 45 was a marine-type, slow-revving diesel, a Sulzer 12LDA28B with a bore of (hence the 28 in the engine designation) and a stroke of . This gave per cylinder, or for the whole engine. The unit was turbocharged and intercooled and gave at 750 rpm. The engine was of the double bank type with two parallel banks of 6 cylinders, geared together to a single output shaft. Six-cylinder versions of the engine were fitted in the Class 25 locos (amongst others) and eight-cylinder versions in Class 33s. Class 45s were the updated versions of the Class 44 locomotives, the latter having a non-intercooled version of the same engine; i.e. the 12LDA28A. The later Class 47 had a modified version of the same engine, a 12LDA28C.
When initially put into service, the locomotives were fitted with multiple-unit working and steam-heating boilers for passenger service. In the early 1970s, fifty were fitted with electric train supply in place of their steam-heating boilers and assigned to work services on the Midland Main Line from London St Pancras to Nottingham, Derby and Sheffield. These locomotives were renumbered as Class 45/1.
The Class 45 is unusual in having a 220 volt electrical system for driving auxiliary systems and battery charging. Most British Railways diesels of the same era had 110 volt auxiliaries.
The Class 45s became the main traction on the Midland Main Line from 1962 and their introduction allowed considerable acceleration of the previous steam-powered service. The Class 45s remained the main source of power on the line up to 1982, when they were relegated to secondary services following introduction of HSTs on the route. From 1986, Class 45s virtually disappeared from the line. From the early 1980s until their withdrawal , the class were regular performers on the North Trans-Pennine line working services from Liverpool Lime Street to York, Scarborough or Newcastle via Manchester Victoria, Huddersfield and Leeds. These trains were usually formed of early Mark 2 carriages, of up to seven in a typical train.
The great majority of Class 45s were withdrawn between 1981 and 1988, after Class 43 HSTs were introduced to their routes; the last was withdrawn from service by 1989.
26 Class 45s were officially named by BR after various British Army regiments and the Royal Marines. Additionally, several were unofficially named.
Eleven locomotives survive in preservation, with examples from both batches. The majority of the preserved engines were built at Crewe Works; in summary:
Mainline Railways introduced OO gauge Class 45s in 1983; D49 The Manchester Regiment and D100 Sherwood Forester in BR green and 45048 The Royal Marines in BR blue.