An articulated type of locomotive designed by Robert F Fairlie in 1865. The design was popular on narrow gauge lines. The frame was mounted on to one or two engine units which could move independently. The engine units could have leading and trailing trucks. A double Fairlie had two engine units, a single Fairlie one, the other being replaced by a bogie. The double Fairlie appeared to have two boilers but this was not the case. They had a single boiler with a firebox and cab in the middle and a smokebox at each end. The cab was in the middle, and the boiler barrel ran right through it, with the driver on one side and the fireman on the other. The driver on a double Fairlie controlled the water supply to the boiler as well as driving, leaving the fireman to look after the fire. This saved space on the fireman's side of the cab. Over 500 Fairlies were built in the USA including, in 1871 the Mason-Fairlie locomotive. They were also used in Russia, India, Australia, and New Zealand. The Fell locomotives used in New Zealand were Fairlies.