Under the Whyte notation for the classification of steam locomotives, 2-6-0 represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, six powered and coupled driving wheels on three axles, and no trailing wheels. This arrangement is commonly called a Mogul.
In the United States of America (USA) and Europe the 2-6-0 wheel arrangement was principally used on tender locomotives. This type of locomotive was widely built in the USA from the early 1860s to the 1920s.
Although examples were built as early as 1852–53 by two Philadelphia manufacturers, Baldwin Locomotive Works and Norris Locomotive Works, these first examples had their leading axles mounted directly and rigidly on the frame of the locomotive, rather than on a separate truck or bogie. On these early 2-6-0 locomotives the leading axle was merely used to distribute the weight of the locomotive over a larger number of wheels. It was therefore essentially an 0-8-0 with an unpowered leading axle and the leading wheels did not serve the same purpose as, for example, the leading trucks of the 4-4-0 American or 4-6-0 Ten-Wheeler types that, at the time, had been in use for at least a decade.
The first American 2-6-0 with a rigidly mounted leading axle was the "Pawnee", built for heavy freight service on the Philadelphia and Reading Rail Road. In total, about thirty locomotives of this type were built for various American railroads. While they were generally successful in slow, heavy freight service, the railroads that used these first 2-6-0 locomotives didn't see any great advantages in them over the 0-6-0 or 0-8-0 designs of the time. The railroads noted their increased pulling power, but also found that their rather rigid suspension made them more prone to derailments than the 4-4-0 locomotives of the day. Many railroad mechanics attributed these derailments to having too little weight on the leading truck.
The first true 2-6-0s were built in the early 1860s, the first few being built in 1860 for the Louisville and Nashville Railroad. The new design required the utilization of a single-axle swiveling truck. Such a truck was first patented in Great Britain by Levi Bissell in May 1857.
In 1864 William S. Hudson, then the superintendent of Rogers Locomotive and Machine Works, patented an equalized leading truck that was able to move independently of the driving axles. This equalized suspension worked much better over the uneven tracks of the day. The first locomotive built with such a leading truck was likely completed in 1865 for the New Jersey Railroad and Transportation Company as their number 39.
It is likely that the locomotive class name derives from a locomotive that was named "Mogul", built by Taunton Locomotive Manufacturing Company in 1866 for the Central Railroad of New Jersey. However, it has also been suggested that, in England, it derived from the engine of that name built by Neilson and Company for the Great Eastern Railway in 1879.