A Triplex locomotive is a steam locomotive that divides the driving force on its wheels by using three pairs of cylinders rigidly mounted to a single locomotive frame. Inevitably any such locomotive will be articulated. All the examples that have been produced have been of the Mallet type but with one extra set of driving wheels under the tender.
Baldwin Locomotive Works built three 2-8-8-8-2 locomotives for the Erie Railroad between 1914 and 1916. One 2-8-8-8-4 was also built, for the Virginian Railway in 1916. This was class XA, so named due to the experimental nature of the locomotive.
The center set of cylinders received high-pressure steam. The exhaust from these was fed to the two other sets of cylinders, which were valved for low pressure. The right cylinder exhausted into the front set of low pressure cylinders, and the left into the rear set; this is also why the high pressure cylinders have the same diameter as the low pressure ones, whereas most mallet locomotives have much smaller high pressure cylinders. The front set exhausted through the smokebox and the rear set exhausted first through a feedwater heater in the tender and then to the open air through a large pipe, which can be seen in the photo. Since only half of the exhaust steam exited through the smokebox, firebox draft (and thus boiler heating) was poor. Although the boiler was large (in line with contemporary two-cylinder and four-cylinder practice), six large cylinders demanded more steam than even such a boiler could supply.
With all six cylinders operating at their full pressure (which could not be sustained for very long), the Triplexes produced huge amounts of tractive effort (TE) that may have been the highest of any steam locomotives before or since. (Westing gives a figure of 160,000 pounds-force (710 kN) in compound mode and seems to indicate that it was the largest TE for any locomotive up to the time [1914-1916]. See the Tractive effort article for other TE figures.) The Triplexes could also be considered the largest tank engines ever built since the tender had driving wheels as well and thus contributed to traction. The problem of variable adhesion on the tender unit was not a serious one, since pusher locomotives had frequent opportunities to take on additional fuel and water.
- Westing 1966, pp. 124–125.
- Leslie Paxton and David Bourne, Locomotive of the South African Railways, Struik, 1985, pp.8-9.
- Westing, Frederick (1966), The locomotives that Baldwin built. Containing a complete facsimile of the original "History of the Baldwin Locomotive Works, 1831-1923", Crown Publishing Group, LCCN 66025422, ISBN 978-0-517-36167-2,.