The Fairlie is a type of locomotive developed in the 1860s by Robert F. Fairlie. While Fairlie's design ultimately proved to have some problems, it had a number of prescient features which were integrated into later locomotive designs. Some modern locomotives owe some of their details to the Fairlie, and researchers have suggested that if the Fairlie had run on a different fuel source, it might have proved to be more viable.
One of the key distinguishing features of the design was the fact that it was double ended. Previous locomotive designs had a clear front and back, and could not be driven backward. As a result, turn tables were needed to turn locomotives so that the direction of a train could be reversed. With a Fairlie, it didn't matter which way the locomotive faced, and this double-ended design is seen in many modern locomotives.
Fairlie's design utilized steam power, the only power available at that point, with separate boilers and fireboxes on either end of the locomotive. He eliminated the tender from his design, arguing that it added unnecessary weight, although this created a problem with fuel availability, as the locomotive could not carry very much fuel without the aid of a tender. Had the Fairlie been powered by a different source, it might have proved to be an enduring design, but the fuel limitations made it an unpopular choice.
The design also featured two sets of trucks, also known as bogies, with the drive wheels attached. One advantage to this design was that it allowed the locomotive to navigate narrow gauge track, but the wheel design also made the locomotive prone to derailment, and it was often criticized for running roughly on straight track. The use of trucks, with some modifications, continues to this day in locomotive design. In a variation on the basic design, the Single Fairlie, only one part of the locomotive was built, creating room for a tender.
Both designs were primarily used in Wales and parts of South America. In South America, the Fairlie proved to be quite valuable for tracks forced to be very curvy and narrow in remote areas. Today, the only extant and working examples of Fairlies can be seen in Wales, where they are operated on novelty railroads which celebrate the age of steam power. People who are interested in the history of railroading find Fairlies interesting from a historical perspective, and some enjoy an opportunity to see one in action.