The LSWR Class T7 4-2-2-0 was a prototype express

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...

design by

Dugald Drummond Dugald Drummond (1 January 1840 – 8 November 1912) was a Scottish steam locomotive engineer. He had a career with the North British Railway, LB&SCR, Caledonian Railway and London and South Western Railway. He was the older brother of the eng ...

for the

London and South Western Railway The London and South Western Railway (LSWR, sometimes written L&SWR) was a railway company in England from 1838 to 1922. Originating as the London and Southampton Railway, its network extended to Dorchester and Weymouth, to Salisbury, Exeter ...

introduced in 1897. Five similar locomotives, classified E10, were introduced in 1901.

Background

Number 720 was a prototype locomotive built in 1897 and classified T7. The layout was unusual and influenced by Francis Webb's 3-cylinder

compound locomotive A compound locomotive is a steam locomotive which is powered by a compound engine, a type of steam engine where steam is expanded in two or more stages. The locomotive was only one application of compounding. Two and three stages were used in shi ...

s introduced in 1883 on the

London and North Western Railway The London and North Western Railway (LNWR, L&NWR) was a British railway company between 1846 and 1922. In the late 19th century, the L&NWR was the largest joint stock company in the United Kingdom. In 1923, it became a constituent of the Lo ...

(LNWR) that employed two pairs of uncoupled driving wheels; the Drummond locomotives were always known as the "double singles". Five similar locomotives, numbers 369-373, were built in 1901 and classified E10.

Design features

Throughout locomotive history, this type of layout with independent uncoupled trains of driving wheels mounted on a common rigid chassis has been repeatedly tried with various aims in mind (the best-known more recent example is the

Duplex locomotive A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive. The concept was first used in France in 1 ...

Aims

In the case of Drummond, the main motive appears to have been to obtain maximum grate area in a period where low-pitched boilers were the norm and the firebox had to be set low between the frames. This limited the width of the grate whilst its length depended on the distance between the coupled axles minus the throw of eventual inside cranks; at the same time there was a reluctance to make the coupling rods too long due to concern about material resistance, for a broken coupling rod flailing away under a locomotive could wreak tremendous havoc. One way out of the impasse was to eliminate the coupling rods altogether and to have two independent pairs of driving wheels each pair driven by its own cylinders.

Earlier experiments

The first engineer known to have adopted this solution was Francis Webb, followed by

Alfred de Glehn Alfred George de Glehn (15 September 1848 – 8 June 1936) was a notable English-born French designer of steam locomotives and an engineer with the Société Alsacienne de Constructions Mécaniques (SACM). His steam engines of the 1890s combined el ...

in France who initially combined divided drive and independent driving axles, finally opting solely for the former whilst coupling the driving wheels. The aforementioned engineers locomotives were compounds and the layout was also a way of separating high-pressure from low-pressure drive trains.

Drummond's locomotives

;Boiler Drummond's T7 and E10 worked with

simple expansion A compound engine is an engine that has more than one stage for recovering energy from the same working fluid, with the exhaust from the first stage passing through the second stage, and in some cases then on to another subsequent stage or even st ...

so that the principle benefit sought would be the increased grate area. Compared with Drummond's standard boilers, the T7's barrel length was increased from 10 feet 6 inches to 12 feet. ;Valve gear Another unusual feature of the T7 and E10 locomotives was the valve gear. The valves for the inside cylinders were operated by

Stephenson valve gear The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for various kinds of steam engines. It is named after Robert Stephenson but was invented by his employees. ...

and the valves for the outside cylinders were operated by Joy valve gear.

Rebuild

In 1905, number 720 was rebuilt with a larger boiler with the diameter increased from 4 foot 5.5 inches (1.4 m) to 4 foot 10.75 inches (1.5 m). The E10s were not rebuilt.

Construction history

* T7, 1 locomotive built 1897, number 720 * E10, 5 locomotives built 1901, numbers 369-373

References

{{SR Locomotives T07 4-2-2-0 locomotives Railway locomotives introduced in 1897 Duplex locomotives Standard gauge steam locomotives of Great Britain