| Power Classification | Unclassified |
| Introduced | 1840 – 1842 |
| Designer | Gooch |
| Company | GWR |
| Weight | 24t 4cwt |
| Driving Wheels | 7ft 0ins |
| Boiler Pressure | |
| Cylinders | 15 in x 18in |
| Tractive Effort | |
| Valve Gear |
The Fire Fly was a class of broad gauge 2-2-2 locomotive used for passenger services on the Great Western Railway. It was the first locomotive designed by Daniel Gooch, first locomotive superintendent at the Great Western Railway.
The class which consisted of 60 locomotives was introduced into service between March 1840 and December 1842, and withdrawn between December 1863 and July 1879.
Fire Fly was one of six built in 1840 by Jones, Turner and Evans of Newton-le Willows who had started up in business three years earlier and acted as subcontractor to Edward Bury and Robert Stephenson. Business tailed off again, and the company closed down in 1852. The works were leased by the London & North Western Railway, who then bought it outright in 1860, forming the nucleus of the Earlestown railway works.
Following the success of the Star class locomotives introduced to the Great Western Railway by Daniel Gooch, Gooch set to work to develop a new class based on North Star, but with larger boilers. The result was the Fire Fly, later followed by 61 similar locomotives designated the same class.
From about 1865, the Fire Fly Class locomotives became part of the Priam Class, along with the Prince class locomotives.
One of the class hauled the first royal train, taking Queen Victoria from Slough to London in 1942.
One advantage of the broad guage locomotives was that they could travel at much faster speeds than those made previously. The Fire Fly class handled the principal trains from Paddington to Bristol when they were new and were capable of hauling trains weighing 80 tons at speeds of up to 60mph.The original Firefly is said to have covered the 30.75 miles (49.49 km) from Twyford to Paddington in 37 minutes, an average speed of 50 miles per hour (80 km/h), which was unprecedented in 1840. It was withdrawn from service in 1870.
A replica of the original Fire Fly was complete by the Fire Fly Trust and unveiled to the public at Didcot Railway Centre in 2005. In 2013 it was given to the Great Western Society.
It has been in steam on regular occasions throughout the year at Didcot Railway Centre but was taken out of service in late 2014 when the boiler certificate expired.
| Home Base | Current Status | Owner |
| Didcot Railway Centre | On static display | Great Western Society |
Preserved British Steam Locomotives 