Mason Bogie

The Mason Bogie is a type of articulated steam locomotive suited for sharp curves and uneven track, once commonly used on narrow gauge lines in the United States. The design is a development of the single Fairlie, and is sometimes, and perhaps more properly known as the Mason Fairlie.

The American licensee of the Fairlie Patent steam locomotive was the firm of William Mason, located in Taunton, Massachusetts. It became obvious that, for all the Fairlie locomotive's advantages, its disadvantages outweighed them. Mason developed an improved design, called the Mason-Fairlie, or more commonly the Mason Bogie (the word bogie is the British word for truck in the railroad sense). Similar locomotives developed in England were known as Single Fairlies.

Mason's idea was to remove what American railroad men saw as the biggest disadvantages of the Fairlie - its cramped space for fuel and water caused by its double ended design (not very useful on American railroads where there was always ample room for a turntable or wye), its cramped cab caused by the joined double boilers, and to some degree its poor riding.

He did this by removing one boiler of the double Fairlie and retaining only one power truck at the front. A much larger cab was fitted, and a fuel bunker and water tank behind the cab, supported by a trailing truck. The advantages of the Fairlie design were kept; the swivelling driven truck for a greater ability to negotiate curves, and the large open space between the trucks to fit a large firebox unrestricted by the wheels.

The Mason Bogie was still, though, plagued by one of the biggest problems of the Fairlie - the jointed steam pipes to the driven truck leaked far too much steam. Mason eventually changed to a different scheme, in which the pivot point for the leading truck became a hollow ball joint through which the live steam for the cylinders passed. 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 valuable space in 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 atop the boiler, with a long lifting link dropping down to the radius rod, a feature unique to Mason Bogies (this was necessary because the lifting link would swing to the side as the truck pivoted, lifting the radius rod and changing the valve setting. Lengthening the link, and thus increasing the radius of its swing, minimized the amount of change.).

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