As I wrote in the post 'Single-Wheeler' locomotives, early locomotives for passenger trains tended to have just a single-pair of driving wheels. This made the mechanism simpler and more free-running. Locomotives like 'Rocket' and the 'Planet' class had a single pair of moderately-large driving wheels to achieve fairly high-speed running on passenger trains. There's a little more in the post 'Planet' in Perspective, which outlines the use of four coupled wheels for freight locomotives where load haulage was more important than speed. There's much more about the history of the 'Planet' class (and the modern replica locomotive at the Museum of Science and Industry in Manchester) here.
Later designers used even larger driving wheels to achieve higher speed without increasing piston speed. But large driving wheels required great skill on the part of the driver to get trains started under less than ideal conditions without excessive slipping.
Patrick Stirling of the Great Northern Railway remained a staunch advocate of 'Singles', such as his famous class of 4-2-2 engines with 8 foot 1 inch diameter driving wheels, introduced in 1870.
Preserved Stirling 4-2-2 displayed at Doncaster in 2003 (Photo: Our Phellap).
When Dugald Drummond was at the Caledonian Railway, he too produced very successful 'Singles', like the 4-2-2 '123' with 7 foot drivers built in 1886.
Preserved Drummond 4-2-2 '123' (Photo: Glasgow Museums).
The growth of passenger demand meant that there was continual pressure to increase the loads hauled and many locomotive designers turned to four-coupled designs, where the torque delivered to the two wheels of the driving axle by the connecting rods was shared, through coupling rods, with a similar pair of wheels which became 'coupled wheels'.
The locomotive designer Patrick Stirling was not convinced, famously claiming, in his Scottish accent, that "A coupled engine is like a laddie running with his breeks [trousers] down". Certainly, the success of coupling wheelsets together depends upon the accuracy with which the crankpins are set, referred to as 'quartering' - inaccuracies will produce a 'stiffness' in the mechanism which may be tolerable in a freight engine not expected to travel at high speed but is unacceptable in an express design.
In the early days of locomotive building there were problems in achieving the desirable accuracy but by 1896, when Johnson produced his handsome '115' class 'Singles' for the Midland Railway, production techniques had evolved sufficiently for the four-coupled passenger locomotive to have become common. Johnson's later designs for the Midland featured the 4-4-0 wheel arrangement which persisted for express locomotives on the Midland and L.M.S. until the Stanier era.
Dugald Drummond (1840-1912) was born in Ardrossan and served an engineering apprenticeship in Glasgow before gaining further exoerience on Scottish railways. After managing the Birkenhead boiler shop of Thomas Brassey, in 1864 he moved to Cowlairs railway works under Samuel Waite Johnson (later of the Midland Railway). He then became Foreman Erector at the Inverness works of the Highland Railway under William Stroudley. When Stroudley moved to the L.B.S.C.R., Drummond followed as his assistant before taking up the post of Locomotive Superintendent of the North British in 1875. In 1882 Drummond moved to the Caledonian Railway before an unsucessful private locomotive building venture in Australia, followed by a time back in Scotland at the Glasgow Railway Engineering Company. He was Locomotive Engineer (later titled Chief Mechanical Engineer) of the London and South Western Railway from 1895 until his tragic death in 1912. He appears to have been outspoken and difficult throughout his career but is recognised as an important locomotive engineer.
His younger brother, Peter, also had a significant career as a locomotive designer.
Picture of the older Dugald Drummond (Wikipedia).
The Drummond 'T9'
In the 'C8' class, Drummond designed an inside-cylinder 4-4-0 with 6 foot 7 inch diameter coupled wheels and a 9 foot coupled wheelbase, similar to earlier designs he'd produced for the North British and Caledonian railways. The 'T9' which immediately followed was intended to increase steaming capacity by lengthening the coupled wheelbase to 10 feet, allowing a larger grate. In Scotland, Drummond had learnt the value of rugged, reliable construction and the 'T9' embodied these principles with 1 inch thick steel frames. Between 1899 and 1901, 66 engines were built in three batches, paired with either a 6-wheel or 8-wheel 'Watercart' tender. Early engines featured a narrow cab and in addition to normal splashers on the coupled wheels which incorporated sandboxes, smaller splashers were provided to clear the coupling rods and crankpins. The third batch of engines built had wider cabs with matching splashers, removing the need for coupling rod splashers and conventional sandboxes were substituted. Vacuum brakes were standard (although at least two had air brakes as well).
'T9' number 307 as originally built and running with a 6-wheel tender seen leaving Waterloo with a Southampton Boat Train, around 1902 (Photo: NRM Nine Elms Collection: Creative Commons)
Drummond had acquired from Stroudley a dislike of unnecessary boiler piercings so the two lock-up safety valves were placed atop the dome - a very distinctive feature. Drummond was not enthusiastic about superheating but he did increase the boiler working pressure on the 'T9' to 175 p.s.i. Enginemen were then encouraged to adjust the 'cut off' to achieve best economy by using the steam expansively. Drummond was an "engineman's engineman", familiar with work on the footplate, although his demeanour could be fierce. He gave lectures to footplatemen at a number of sites. These lectures were later published as a pocket reference book.
The Drummond arrangement of Stephenson Link Motion operating slide valves was regarded as particularly successful and free-running. The fitting of a steam reverser made frequent adjustments to the 'cut off' an easy task.
Eastleigh drawing E34 below shows the original appearance of the first batch of twenty locomotives, built at Nine Elms.
Eastleigh drawing E34: 'T9' (shows 8-wheel tender).
Click here for larger view.
Although Drummond did not embrace superheating, he was anxious to maximise steam generation and he introduced various water tube arrangements to his boilers. The second batch of thirty 'T9', built by Dubs (there's a short history of Dubs here) incorporated two groups of water cross tubes in the firebox. With or without water tubes, the class performed well.
In 1922, Urie rebuilt number 314 with an Eastleigh pattern superheater, extended smokebox and stovepipe chimney. Subsequently, the rest of the class received these changes, generally giving the appearance shown in Eastleigh drawing E34B below. After 1925, the Maunsell superheater replaced the Eastleight pattern.
Eastleigh drawing E34B: 'T9' as rebuilt (shows 6-wheel tender).
Click here for larger view.
The T9 was liked by enginemen and came to be regarded as Drummond's 'masterpiece'. The class had a long life - none were withdrawn until the 1950s and the last (number 120 from 1899, renumbered 30120 by British Railways) survived until 1963 when it became part of the National Collection managed by National Railway Museum.
On Saturday 8th April 2017, I finally got to drive the National Collection's 'T9' at the Battlefield Line. But that's another story.
Drummond 'T9': 30120 being prepared at Shackerstone on 8th April 2017.
 ‘Drummond Locomotives – A Pictorial History’ by Brian Haresnape and Peter Rowledge (Ian Allen 1982) ISBN 0 7110 1206 7.
 ‘A Pictorial Record of Southern Locomotives’ by J. H. Russell (Haynes Publishing 1991).
 ‘Great Locomotives of the Southern Railway’ by O. S. Nock (Guild Publishing).
Related posts on other sites
Dugald Drummond (Wikipedia).
Dugald Drummond (Grace's Guide).
Dugald & Peter Drummond (Steam Index).
Related posts on this site
'Planet' in Perspective.
Where necessary, clicking on an image above will display an 'uncropped' view or, alternately, pictures may be selected, viewed or downloaded, in various sizes, from the album listed:-
[Old picture added, corrections: 17-Apr-2017]