British Rail Class 23 (Baby Deltic) diesel-electric locomotives

Overview

The British Rail Class 23 was a compact class of ten diesel-electric locomotives introduced in 1959. Originally designated the English Electric Type 2, the ten members were numbered D5900–D5909. They were diesel-electric machines intended to provide medium-power traction for a variety of passenger and freight duties and were notable for their use of a Napier Deltic power unit.

Design and engineering

Mechanically the Class 23 followed the diesel-electric pattern of a diesel prime mover driving a generator that supplied current to axle-mounted traction motors. Each locomotive used a Napier Deltic T9-29 nine-cylinder engine rated at about 1,100 bhp which drove an English Electric generator; that generator in turn fed four traction motors. The locomotives employed a Bo-Bo wheel arrangement (four axles in two bogies) and were built by the English Electric company.

Powerplant details

The Deltic engine family is characterized by a distinctive triangular layout of cylinder banks and opposed-piston construction; the Class 23’s straight-nine variant is a smaller member of that family. Its compact, high power-to-weight design inspired the nickname "Baby Deltic", reflecting a resemblance to the much larger Class 55 Deltic locomotives. The traction equipment and motors were supplied in line with contemporary English Electric practice and are described in sources on diesel-electric traction (traction motors).

Service history and performance

Introduced into traffic in the late 1950s, the Class 23s were used on mixed traffic and secondary express workings where a compact, medium-powered locomotive was required. While the Deltic engine offered high specific power for its size, these locomotives experienced operational and reliability limitations compared with larger classes, and their service careers were comparatively short. They were part of the broader modernization period in British rail traction represented by the diesel-electric transition from steam.

Legacy and context

Although only ten units were built, the Class 23 are of interest to rail historians and enthusiasts because they illustrate an experimental approach to matching a novel engine type and compact design to mainline requirements. They occupy a distinct place in mid-20th-century British diesel development and are often mentioned in discussions of how engine selection and maintenance affected small classes.