Geared steam locomotive
This gearing is part of the machinery within the locomotive and should not be confused with the pinion that propels a rack locomotive along the rack between the rails. The geared steam locomotives that have been built have been for conventional track, relying on the adhesion between wheels and rail.
Steam locomotive
More-powerful locomotives tend to be longer, but long rigid-framed designs are impractical for the tight curves frequently found on narrow-gauge railways. Various designs for articulated locomotives were developed to overcome this problem. The Mallet and the Garratt were the two most popular, both using a single boiler and two engines (sets of cylinders and driving wheels). The Garratt has two power bogies, whereas the Mallet has one. There were also a few examples of triplex locomotives that had a third engine under the tender. Both the front and tender engines were low-pressure compounded, though they could be operated simple (high-pressure) for starting off. Other less common variations included the Fairlie locomotive, which had two boilers back-to-back on a common frame, with two separate power bogies.