British Rail Class 21 (North British Type 2)

Design and characteristics

The Class 21 were diesel-electric locomotives in the general Category of diesel locomotive. They followed the Type 2 power classification, meaning they were intended for medium-powered duties such as local passenger services and freight. The layout combined a diesel prime mover with electric transmission driving the wheelsets. Externally the locomotives had a utilitarian appearance typical of late 1950s British diesels.

Service history and problems

Introduced during the post-war modernization programme, the class quickly gained a reputation for poor reliability. Mechanical and electrical faults limited availability and increased maintenance costs. These problems shortened the expected service lives of many members of the class; a minority were rebuilt with different engines in an attempt to improve performance and were reclassified as a separate type.

Uses, withdrawal and legacy

When operational they performed local passenger and secondary freight duties, but recurring failures meant that many units were withdrawn or scrapped earlier than contemporary designs. A small number were re-engined and reclassified, reflecting efforts to salvage useful machines rather than write them off immediately. The troubled history of the class contributed to lessons learned about procurement, standardization and factory quality control in the early diesel era.

Later reuse of the class number

Under the TOPS numbering system the designation "Class 21" was later reused in the 21st century for a different family of locomotives: the Vossloh G1206 type acquired by some operators from around 2007. See the modern usage as Vossloh G1206 for distinct, later machines. For further historical context on the original North British machines consult records and preservation sources linked by national archives and enthusiast groups.

• Builder: North British Locomotive Company
• Place of construction: Glasgow
• Operator: British Railways
• Common name: North British Type 2

For more technical and historical detail see specialist publications or archival material on mid-20th-century British diesel traction. General introductions to the diesel era and individual class histories can aid comparison between the Class 21 and contemporaneous designs.