The first 19 locomotives ordered by Isambard Kingdom Brunel for the Great Western Railway included two unusual Haigh Foundry locomotives.

GWR Snake and Viper South Devon Railway Exe and Teign
Snake rebuilt as 2-2-2T locomotive ca. 1865
Type and origin Power type Steam Builder Haigh Foundry Serial number 25–26 Build date 1838 Total produced 2
Specifications Configuration: ​  • Whyte 2-2-2 2-2-2T (after rebuild) Gauge 7 ft 1⁄4 in (2,140 mm) Leading dia. 3 ft 5 in (1,041 mm) Driver dia. 6 ft 4 in (1,930 mm) 6 ft 0 in (1,829 mm) (after rebuild) Trailing dia. 3 ft 5 in (1,041 mm) Wheelbase 13 ft 0 in (3.96 m) Cylinder size 14.75 in × 18 in (375 mm × 457 mm) 13 in × 18 in (330 mm × 457 mm) (first rebuild) 15 in × 18 in (381 mm × 457 mm) (second rebuild)
Career Operators Great Western Railway South Devon Railway Number in class 2 Withdrawn 1868-1869 Disposition Unknown, probably scrapped

Snake and Viper were built at the Haigh Foundry and delivered in September 1838. They had 14.75 in × 18 in (375 mm × 457 mm) cylinders and the driving wheels geared 2:3 to keep the cylinder stroke speed low while allowing high track speed, in line with Brunel's specifications. The boiler had a diameter of 39 in (991 mm) and was 9 ft (2,743 mm) long.^[1] Both locomotives became only really useful after modifications in the years 1839 till 1840 they had been rebuilt with 13 in × 18 in (330 mm × 457 mm) and conventional drive. Most probably they received their 6 ft (1,829 mm) driving wheels at the same time. They were later converted to 2-2-2T tank locomotives, possibly when they were sent to work the South Devon Railway in 1846, where they got the names Exe and Teign. The locomotives returned 1851. Snake operated till November 1869, Viper till January 1868. The boiler of Viper was afterwards used as stationary boiler in Shrewsbury.^[2]