thumb|300px|The Wm. Mason, an 1874 0-6-6 Mason Bogie and the first locomotive with [[Walschaerts valve gear built in the United States. Note the intricate decorative work.]]
thumb|300px|Boston, Revere Beach & Lynn #6, an 1886 product of Mason Machine Works. This is a 2-4-4 Mason Bogie. Note the deep firebox permitted by this configuration, and the high reversing shaft (below the bell) with a long lifting link to the valve gear radius rod.thumb|300px|2-4-4T Mason Bogie #10. Mason Machine Works, Taunton, MA. Built 1887. Photo location: Behind the Lynn, MA Train Station of the Boston, Revere Beach and Lynn Railroad. Photo Source: Lynn Museum.
Mason Bogie locomotives (also known as Mason Fairlie locomotives) are a type of articulated tank locomotive suited for sharp curves and uneven track, once commonly used on narrow-gauge railways in the United States. The design is a development of the Single Fairlie locomotive.
Concept and development
The American licensee of the Fairlie Patent steam locomotive was the firm of William Mason, located in Taunton, Massachusetts. Mason's first Fairlie locomotive was the Janus, an Double Fairlie built in 1871.
Janus was not commercially successful and was not repeated, so Mason experimented with a different design. In 1869, a Single Fairlie locomotive had been designed and constructed by Alexander McDonnell for the Great Southern and Western Railway in Ireland. This had a single boiler with one articulated, powered truck beneath it and a second, unpowered truck beneath the cab and bunker. Mason developed a set of standard plans based on this design with modified steam delivery systems.
Onward was a largely successful engine, but certain design elements proved troublesome. Bolts worked loose, center castings were too light, and the steam pipes and gaskets were insufficient. Mason also developed a sliding seal for the exhaust from the moving cylinder saddle into the smoke box. Although better, Mason's improvements took up much-needed space between the driving wheels, forcing Mason to use an outside valve gear (generally the Walschaerts valve gear). Additionally, the reversing shaft had to be mounted on top of the boiler, with a long lifting link dropping down to the radius rod, a feature unique to Mason Bogies due to the rotating truck and tight clearances. This was about half the firm's total output of locomotives during this period. Major buyers included the Boston, Revere Beach and Lynn Railroad, a suburban carrier which owned 32, the Denver, South Park and Pacific Railroad, a Colorado common-carrier railroad which owned 23, the New York and Manhattan Beach Railroad which owned 17, and the Wheeling and Lake Erie which was the largest standard gauge customer with 16 locomotives.
The best-known Mason Bogies were those of the Denver, South Park and Pacific, which were the largest built for narrow gauge. Copies of the DSP&P's biggest engines were built for the Denver, Utah & Pacific, which sold them to the Burlington and Northwestern Railway, a CB&Q subsidiary in Iowa. The DSP&P engines were hard-used in Utah and all gone by 1894 except for one, which survived through a chain of owners until 1942.
The Boston, Revere Beach and Lynn Railroad almost exclusively used Mason Bogies and continued to order bogie types after Mason ceased locomotive production in 1890.