LMS 4P Compound

The LMS 4P Compound Class refers to the continuation and extension of the celebrated Midland Railway Compound Class under the London Midland and Scottish Railway, which inherited the Midland Compound 4-4-0 design at the 1923 Grouping and recognised its exceptional efficiency by ordering further examples rather than replacing it. The Compound was a four-cylinder locomotive using the Johnson/Deeley compound system: the steam was expanded first in a pair of small high-pressure inside cylinders and then exhausted into a single large low-pressure outside cylinder on each side, extracting more work from each pound of steam than a simple-expansion locomotive could achieve.

The Midland Compound had been developed by S. W. Johnson from 1902 and refined by Richard Deeley from 1905 into one of the most thermally efficient British locomotive designs of the Edwardian era. The LMS ordered additional examples in 1924–1932 to a slightly modified specification, bringing the total fleet to 195 locomotives — by far the most numerous compound class in British railway history. Under the LMS the Compounds worked a wide range of duties: the Midland main-line expresses between St Pancras and Manchester, Leeds, and Sheffield; the cross-country services to Bristol and the South West; and, in their later years, secondary passenger duties across the former-Midland network.

The original Midland Railway Compound No. 1000 is preserved at the National Railway Museum, York, where it represents both the peak of Midland Railway locomotive design and one of the most significant compound locomotive types in British history.