The British Rail Class 45 or Sulzer Type 4 are 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 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 Midland Main 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 From 1986 Class 45s virtually disappeared from the line and all were withdrawn by the end of 1989. All remaining survivors now preserved.