Convertible 0-6-0 locomotive
In September 1864, Daniel Gooch resigned as Locomotive Superintendent. Joseph Armstrong, who was in charge of the Great Western Railway's (GWR) Wolverhampton works, replaced Gooch as Locomotive Superintendent until Armstrong's death in 1877, when his assistant, William Dean, replaced him.
By 1864, the end of the broad gauge was in sight and only a further 70 purely broad gauge locomotives were built. From 1876, most locomotives which were intended to run on the broad gauge were "convertibles", being capable of alteration to run on standard gauge track by moving the wheels from outside to inside the frames.
26 locomotives with 6ft driving wheels built in 1865/6 for passenger and general traffic. 10 were converted to saddle tanks with 5ft driving wheels in 1877 and worked on branch lines. 17 survived until the end of the broad gauge, including Hedley which survived as a stationary engine at Neath until 1929.
14 locomotives with 5ft driving wheels built in 1865/6 for goods traffic. All were sold to the Bristol & Exeter Railway in 1873/4, but reverted to GWR ownership when the B&ER was amalgamated in 1876. Only one survived until May 1892, the remainder being scrapped between 1887 and 1891.
Six locomotives with 4ft 6in driving wheels built in 1865/6 to work goods traffic on the Metropolitan Railway. They were the only pure broad gauge locomotives to be built with side tanks and the only ones with round domes (except some of the "freak" locomotives built in the 1830s). Three were sold to the South Devon Railway in 1872 and two survived until 1892.
24 locomotives with 8ft driving wheels built between 1871 and 1888. These were officially renewals of Gooch's Iron Duke Class of 1847 and the first six (built between 1871 and 1873) may have retained parts of the original locomotives. These locomotives worked the major express trains on the broad gauge between London and Exeter or Newton Abbot. Tornado was the last GWR broad gauge locomotive to be built in July 1888 and had a working life of less than four years.
In 1876 Armstrong introduced the first 10 "convertible" locomotives. These were 0-6-0 saddle tanks with 4ft 6in driving wheels. In 1877/8, under Dean, a further 40 were built, five as broad gauge locomotives and 35 as standard gauge locomotives that were briefly converted to broad gauge in the late 1880s to replace old stock. All 50 locomotives were converted to standard gauge between May 1892 and March 1893.
Between 1884 and 1888, 20 of Armstrong's "standard goods" 0-6-0 tender locomotives, built in 1876 as standard gauge locomotives, were converted to broad gauge, reverting back to standard gauge in 1892.
Dean introduced a further 41 convertible locomotives that ran on the broad gauge. Ten were 2-4-0 side-tank locomotives built in 1885 that were later converted to tender engines. Twenty 0-4-2 saddle tank locomotives were built in 1888/9 which were soon converted to 0-4-4 back tank locomotives for work on the South Devon and Cornwall lines. Three 2-4-0 express passenger locomotives built to an experimental four cylinder tandem compound design were built in 1886-1888, but were not a success and were rebuilt as standard gauge 4-4-0 locomotives in 1894. The remaining eight convertible locomotives were 2-2-2 express locomotives built in 1891 to handle the increasing passenger traffic immediately prior to the end of the broad gauge.