A Double Fairlie is a steam locomotive built to the Fairlie's Patent design of Robert Francis Fairlie using two articulated power bogies, one at each end of the engine. The Ffestiniog Double Fairlies are all of an 0-4-4-0 wheel arrangement.
The FR is famous as the home of the Double Fairlies. The first design of narrow gauge Double Fairlies were built for the 3ft 6in gauge in Queensland in 1866 - designed by Charles Fox. They contained some unique features and were not successful. The next design was built for the FR by George England & Co. On the narrower gauge it was a great success. It saved the FR from having to go to the considerable expense of doubling the line and provided the impetus for narrow gauge railway building across the world.
The Double Fairlie went on to enjoy considerable if generally short-lived success in various countries, notably Mexico and the Caucasus.
 Ffestiniog Double Fairlies
No. 7, Little Wonder, built by George England & Co in 1869
No. 8, James Spooner, built by Avonside Engine Company in 1872
Source: Boyd, The Festiniog Rly, Vol 2
 FR Fairlie Power Bogies
The power bogies for the Ffestiniog's Fairlies (doubles and single) have some standard features but are not interchangeable:
- Tal's is different from the double engines due to the provision of sanders. It was built around 1997
- Merddin's two bogies and Tal's single have steam-operated drain cocks, but DLG and EoM have lever operated drain cocks.
- DLG's bogies are the oldest pair still operating. The frames are from Livingston Thompson and date back to 1896 if not before. They were in use under Merddin Emrys from 1934 until 1972, and then, much altered, under the new EoM until 1992. As a further confusion the original Taliesin cylinders were inserted in one of these bogies in 1936!.
- Earl of Merioneth's bogies were made new in 1985-8 for Merddin Emrys and put under the Earl in 1996
- Merddin's current bogies were new in 2004-2005.
- Livingston Thompson's current bogies are the frames & cylinders from the original Merddin Emrys (used under Taliesin II/ Earl Of Merioneth from 1932 until 1972) and the rods and wheels from James Spooner
[Sources: FR Magazine 157 - Jo Clulow - Musical Bogies, & Tony Willmore, WHR e-Group posting 22681 ]