Vintage photo of a coal-burning, triplex steam locomotive built by the Baldwin Locomotive Works in 1914.
This is a class P-1 engine with the very unusual wheel configuration of 2-8-8-8-2. The unit was used as Erie Railroad No. 2603, nicknamed "Matt H. Shay". Click to enlarge.
(James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)
2-8-8-8-2 ERIE TRIPLEX
2-8-8-8-2 Mallet triplex steam locomotive No. 2603 was built by the Baldwin Locomotive Works for the Erie Railroad Company, circa 1913-14.
A model of Erie Baldwin Triplex locomotive No. 2603 from 1913. The model was made by Ernest "Mooney" Warther. Click to enlarge.
(Kaufman Color Service, Orville, OH, Public domain, W. Lenheim Collection)
Baldwin Locomotive Works built three Triplex locomotives for the Erie Railroad between 1914 and 1916. The first was named Matt H. Shay, after a beloved employee of that road. These Triplexes were given the classification of "P1" and they could reportedly pull 650 freight cars. The triplexes were primarily used as pushers on grades requiring helper locomotives. Slow moving, the triplexes were not considered highly successful, and no more were built for Erie. The Erie Railroad scrapped their Triplexes from 1929, 1931, and 1933.
Purpose
The purpose of the triplexes was banking heavy trains over steep inclines, requiring high tractive effort, but low speed, about 10 mph (16 km/h), over short distances.
The center set of cylinders received high-pressure steam. The exhaust from these was fed to the two other sets of cylinders, which had valves designed 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, making the engine a 2 to 1 compound, 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.
The Erie locomotives always operated compound and did not have starting valves which would have put full pressure on all six cylinders, even so 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 lbf (710 kN) in compound mode and seems to indicate that it was the largest TE for any locomotives up to the time (1914–1916). The triplexes could also be considered the largest tank locomotives 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. In total, only four triplexes came into existence and only in the United States; all of the Erie triplexes were retired by 1930; none were preserved.