Triplex Locomotives.

Three times the locomotive.

Updated: 19 Sept 2021

First Matt H Shay pic improved

Previous update: 24 Nov 2002
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The triplex locomotive was intended to use the considerable weight of the tender to increase the hauling power of big locomotives. This did not end well, because it was very difficult to generate enough steam to supply three sets of driving apparatus, and because the weight of the tender was reduced as coal and water were used. How to differentiate a triplex from a steam tender locomotive? The former has three driving systems, rather than two.


Above: The Matt H Shay, a triplex used for heavy freight haulage on the Erie RR. Note steam exhaust pipe at rear.

On a Mallet triplex, there are three sets of driving wheels, with the front set swivelling under the front of the boiler, the middle fixed in the er... middle, and the rear set mounted under the tender. The wheel arrangement of the compound loco above is 2 - 8 - 8 - 8 - 2. No. 5014 was built by Baldwin to the design of George Henderson, and was intended for use on the Gulf Summit grade. When introduced it was believed to be the heaviest and most powerful locomtive in the world. (the picture above clearly shows the number 2603 on one of the pair of sand domes nearest the cab- this seems to have been the number it operated under.

The HP cylinders are in the centre; the steam from the right-hand HP cylinder was led to the forward LP cylinders, and the from the left-hand HP cylinder back to the tender LP cylinders. The exhaust from the front LP cylinders fed a normal blastpipe in the smoke box. The rear LP exhaust passed through a feedwater heater in the tender before exiting via a 20" diameter vertical pipe at the rear of the tender, clearly visible in the photograph. The initial firegrate area of 90 sq ft proved inadequate and after a year was rebuilt to 122 sq ft.
Two later models 5015 and 5016 were delivered in 1916 and these were slightly modified from the details below. These locos were used in regular service on the Susequhanna incline (1.5%) where they replaced three ordinary pusher locos.

Even with the enormous boiler, supplying enough steam for all those cylinders was a daunting problem, probably because only half of the exhaust steam was used to create boiler draught. The type was not considered a success. No more were built.

Boiler pressure: 210 psi
HP cylinders: 36" diam x 32" stroke (two, in centre)
LP cylinders: 36" diam x 32" stroke (four, two each end)
Weight in service: 853,050 lbs
Adhesion weight: 761,000 lbs
Proportion of weight on drivers: 89%
Wheelbase: 90 feet
Boiler diameter: 7' 10"
Boiler diameter at dome ring: 9' 0"
Tractive force: 160,000 lbs
Wheel diameter: 5' 3"
Firebox length: 13' 6''
Grate area: 90 sq ft
Street mechanical stoker
53-element Schmidt superheater
Baker valvegear
Ragonnet steam reversing gear

Left: The Matt H Shay: 1914

This photograph appeared in Popular Mechanics in July 1914, p27.

It seems to be based on the first picture in this section.


Above: Another Baldwin Triplex, the Virginian 2 - 8 - 8 - 8 - 4. Also not a success.

This engine was ordered to work the difficult 14-mile section from Elmore to Clark's Gap; only one was built. Although the boiler was bigger than that of the Erie triplex, there was still not enough steam. Another problem was steam leakage from the stuffing boxes of the tender cylinder, which obscured vision from the cab, and the machine was rebuilt without the powered tender.

You might think that an inherent problem with these engines- one that would have occurred to the designers- was that as the tender emptied, the adhesion of the rear driving wheels steadily decreased. However, Wiener states that since they were used for banking, there were frequent opportunities to replenish coal and water, and this was not a serious problem.

Boiler pressure: 215 psi
HP cylinders: 34" diam x 32" stroke (two, in centre)
LP cylinders: 34" diam x 32" stroke (four, two each end)
Weight in service: 844,000 lbs
Adhesion weight: 726,000 lbs
Boiler diameter: 8' 4"
Tractive force: 146,000 lbs
Wheel diameter: 4' 8"
Grate area: 108 sq ft
Total wheelbase: 91' 3"

Articulated Locomotives; Lionel Wiener. Constable & Co, 1930
The New Illustrated Encyclopaedia of Railways.

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