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Virginian Railway Class Xa 2-8-8-8-4T Triplex Steam Locomotive
Only one 2-8-8-8-4T was ever built, a Mallet-type for the Virginian Railway in 1916. Built by Baldwin Locomotive Works, it became the only example of their class Xa, so named due to the experimental nature of the locomotive. Like the same railroad's large articulated electrics and the Erie Railroad 2-8-8-8-2s, it was nicknamed "Triplex".

The purpose of the Xa was to push heavy trains over steep inclines, requiring high tractive effort, but low speed, over short distances. The Xa worked on a difficult 14-mile section from Elmore to Clark's Gap.

The Xa's center set of cylinders received high-pressure steam, and 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 Xa produced huge amounts of tractive effort that may have been the highest of any steam locomotive before or since (160,000 pounds-force in compound mode, which was the largest tractive effort for any locomotive up to the time,1914-1916). The Xa is also considered the largest tank engine ever built since the tender had driving wheels as well and thus contributed to traction. The problem of variable adhesion on the Xa's tender unit was not a serious one, since pusher locomotives had frequent opportunities to take on additional fuel and water.

The Xa was unable to sustain a speed greater than five miles an hour, since the six cylinders could easily consume more steam than the boiler could produce. The tender had a four-wheel truck at the rear to help guide the locomotive into curves when drifting back downhill after pushing a train over the hill.

The XA was sent back to Baldwin in 1920 and was rebuilt as two locomotives, a 2-8-8-0 and a 2-8-2. Unlike their progenitor which lasted only a few years in service, these two locomotives remained in service until 1953.

Virginian Class Xa Triplex Overview

Type and origin
Builder: Baldwin Locomotive Works
Build date: 1916
Class: Xa
Number in class: 1
Specifications
Wheel Arrangement: 2-8-8-8-4T
Gauge: 4 ft 8 1⁄2 in (1,435 mm) standard gauge
Driver Diameter: 56 in (4' 8")
High Pressure Cylinders (two in center):  34" x 32" (Diameter x Stroke)
Low Pressure Cylinders (four, two at each end): 34" x 32" (Diameter x Stroke)
Boiler Pressure: 215 psi
Adhesive Weight: 726,000 lbs (325 tons)
Weight in Service: 844,000 lbs (380 tons)
Tractive force: 146,000 lbs
Total wheelbase: 91' 3"
Grate Area: 108 sq ft
Evaporative Surface: 8,120 sq ft
Superheater Surface: 2,060 sq ft
Career
Operator: Virginian Railway
Road number: 700
First Run: 1916
Last Run: 1920
Disposition: Rebuilt as a 2-8-8-0 and a 2-8-2 by Baldwin.

See also:

Steam Locomotives

Triplex Locomotives

Builder's photo of the Virginian Xa, the sole 2-8-8-8-4 locomotive.

Builder's photo of the Virginian Xa, the sole 2-8-8-8-4 locomotive. This locomotive was in service from 1916 to 1920, and this photo was taken somewhere in that time frame. Although it was used exclusively on the Virginian Railway, where it was classed Xa, it's not clear whether Virginian actually purchased the locomotive from Baldwin.

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Wheel arrangement diagram of the VGN Xa. Front of locomotive on left.

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Wheel arrangement diagram of the VGN Xa. Front of locomotive on left.

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