Steam locomotive
Articulated locomotives

Very powerful locomotives tend to be longer than those with lower power output, but long rigid-framed designs are impracticable 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. They had a single boiler and two engine units (sets of cylinders and driving wheels): both of the Garratt's engine units were on swivelling frames, whereas one of the Mallet's was on a swivelling frame and the other was fixed under the boiler unit. A few triplex locomotives were also designed, with a third engine unit under the tender. Other less common variations included the Fairlie locomotive, which had two boilers back-to-back on a common frame, with two separate engine units.
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