LNWR Tank

The LNWR Precursor Tank Class — commonly referred to simply as the LNWR Tank — was a series of 4-4-2T side-tank express passenger locomotives built at Crewe Works from 1906 as the tank engine version of George Whale's Precursor Class 4-4-0, designed for the LNWR's intensive London suburban and outer-suburban services where the Atlantic tank wheel arrangement gave good high-speed performance without the range limitations of a tender engine. Webb had earlier introduced tank versions of his compound types, but the Precursor Tank was a far more successful design, embodying the same practical simplicity and reliability that had made the Precursor 4-4-0 itself such a marked improvement over the Webb compound era.

Whale designed the Precursor Tank with the same inside cylinders and Stephenson valve gear as the Precursor 4-4-0, fitting a generous side tank capacity for the outer-suburban runs on the LNWR's Watford, Tring, and north London services from Euston. The 4-4-2T Atlantic tank wheel arrangement — four leading wheels, four coupled driving wheels, two trailing wheels beneath the rear tank — gave stability at the higher speeds of the outer-suburban runs, while the coupled wheels gave adequate adhesion for the frequent stops of suburban service.

In practice the Precursor Tanks were well-regarded by LNWR crews for their reliability and straightforward handling, qualities that had been noticeably absent during the Webb compound years. They worked the Watford electrification preparatory services and continued on outer-suburban working until electrification displaced them on the busiest London routes. After the 1923 Grouping the LMS inherited the fleet and continued them on former-LNWR suburban duties. Bowen-Cooke built a superheated development of the Precursor Tank, the George V Tank, which gave still better performance. None of the Precursor Tanks was preserved.