The 3521 Class were forty tank locomotives designed by William Dean to haul passenger trains on the Great Western Railway. They were introduced as locomotives in 1887, but were quickly altered to become s to improve their running. Following two serious accidents they were further altered from 1899 to run as tender locomotives, in which form the last was withdrawn in 1934.

Locomotives

0-4-2T standard gauge

The first twenty locomotives were turned out in 1887 as locomotives for services on the lines.

0-4-2ST broad gauge

In 1888 a further batch of twenty were ordered as s for the broad gauge lines in Devon and Cornwall.

0-4-4T broad gauge

Due to the unsteady running of the s, the last of the order, 3560, was turned out in August 1889 as a bogie side tank. The remainder of the class were altered to a similar layout over the following two years. 3560 was slightly different at this time, having a bogie that was 6 in shorter and an overall wheelbase of 20 ft, rather than the 21 ft of the converted locomotives.

0-4-4T standard gauge

The standard gauge s were converted to s in the same manner as the broad gauge locomotives, which were all eventually converted to standard gauge. By the end of 1892 the whole class of forty locomotives was to one standard design for the first time.

4-4-0 standard gauge

All forty locomotives were rebuilt as tender locomotives between 1899 and 1902. Twenty-six locos retained their parallel domed boilers while fourteen received new Standard No 3 parallel domeless boilers (later replaced by the taper barrel version of that type). (Holcroft states that twenty-six received Standard No 3 boilers.) Holcroft was of the opinion that the original inside and outside frames were modified whereas Le Fleming was of the opinion that new inside frames would have been required. However as the modification also increased the coupled wheelbase from 7 ft to 8 ft, it is unlikely that much of the original framing was re-used. Two locomotives, 3521 and 3546, were sold to the Cambrian Railways in August 1921 to replace locomotives destroyed in the Abermule accident. They were allocated Cambrian numbers 82 and 95 respectively but these were never carried, the two locomotives being returned to the Great Western Railway with their original numbers when the two railways were amalgamated at the start of 1922.

Accidents and incidents