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Leading wheel)
On a steam locomotive, a leading wheel or leading axle is an unpowered wheel or axle located in front of the driving wheels. The axle of the leading wheels were usually located on a leading truck. Leading wheels are used to help the locomotive negotiate turns and to hold up the front portion of the boiler. The first use of leading wheels is commonly attributed to the 4-2-0 designs of John B. Jervis in 1832 (a locomotive type that has since become known as the Jervis type).
In the Whyte notation, leading wheels are designated by the first number in the series. For example, the 2-8-2 Mikado type locomotive had two leading wheels, eight driving wheels, and two trailing wheels. Some locomotives such as the 0-10-2 Union type had no leading wheels and were thus designated with a zero in the first position.
In the Whyte notation the number designates the number of wheels rather than the number of axles, thus the first 2 in the Mikado's 2-8-2 refers to two wheels (one axle) while the first 4 in the Northern type's 4-8-4 designation refers to four wheels (two axles).
The highest number of leading wheels on a single locomotive is six as seen on the 6-2-0 Crampton type and the 6-8-6 Pennsylvania Railroad steam turbine locomotives.
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