South African Class FC 2-6-2+2-6-2

Cape Government Railways Double Fairlie, c. 1888

The first Fairlie locomotives later became known as Double Fairlies. In general appearance, most of them were reminiscent of the German-built Zwillinge which saw service on the narrow gauge lines in German South West Africa and other German territories. However, while the Zwillinge were pairs of individual locomotives which were semi-permanently coupled back-to-back, the Double Fairlie was a single locomotive with either a double-ended boiler or two separate boilers with smokeboxes at each end, a central firebox and a central cab, all mounted on one rigid frame. It ran on two swiveling engine units or powered bogies similar to those of a Garratt locomotive, with the cylinders on each engine unit at the respective locomotive ends. Couplers, buffers and, where they were in use, cowcatchers were mounted on the engine units so that they could more accurately follow the curvature of the track.[5][6]

A variation of the Fairlie concept was the Single Fairlie, also known as the Mason Fairlie. The Single Fairlie design was essentially half a Double Fairlie. It retained the ability to negotiate sharp curves and while it abandoned the bidirectional nature of the Double Fairlie, it regained the ability of conventional locomotives to have a large water and coal bunker behind the cab and to use a trailing tender if necessary.[5][6]

Swindon Marlborough & Andover Railway Single Fairlie, c. 1878

Early Single Fairlies were similar in appearance to regular tank loco­mo­tives. The whole engine and bunker or tender combination was mounted on a single rigid frame as on the conven­tional tank engine, but it differed by virtue of the fact that a pivoting engine unit was mounted under the boiler and an unpowered bogie under the bunker or tender.[5][6]

Mason Bogie Single Fairlie, c. 1874

Later models were similar in appearance to conven­tional tender locomotives. It was developed by William Mason in the United States of America, where the type became known as the Mason Bogie. It had one boiler, a cab and bunker at one end, a single engine unit or powered bogie under the boiler and an unpowered bogie under the cab and bunker. The illustration shows a Mason Bogie with an 0-6-6 wheel arrangement.[5][6]

The first Modified Fairlie was introduced in South Africa and was viewed as a competitor and alternative to the Garratt type. Colonel Collins displayed a distinct preference for articulated locomotives and the Modified Fairlie was acquired as an experimental locomotive in 1925 to compare its concept to that of the Garratt locomotive in terms of actual performance and maintenance requirements.[1][2][3]

The Class FC Modified Fairlie was, in essence, an adaptation of the Kitson-Meyer concept. It was similar in appearance to a Garratt, but with the boiler, cab, coal and water bunkers all mounted on a single rigid frame which extended almost over the full length of the locomotive. This superstructure was supported on two engine units by means of pivots. The pivot centres were located approximately at the centre of the coupled wheelbase of each engine unit. In the Garratt design, by comparison, the coal and water bunkers are mounted directly on the engine units and pivot with them, while the boiler, firebox and cab are mounted on a rigid cradle frame which is suspended between the two engine units.[1][2][3][7]

On the Modified Fairlie, as compared to the Double Fairlie, the distance between the engine units had been increased to a figure similar to that of the Garratt type, thereby allowing space between them for a liberal firebox and ashpan. This resulted in a more efficient boiler and also distributed the weight of the engine over a greater length than was obtainable with an orthodox Double Fairlie design. It was claimed that the Modified Fairlie arrangement reduced the wear between wheel flanges and rails and that the distribution of the loads on the axles as fuel and water was expended was less variable than could be obtained with the Garratt design.[2]