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Great Western Railway class 5100 “Large Prairie” and originally class 3100
Great Britain | 1903 | 40 produced
Number 3120, later 51200
Number 3120, later 51200
J.R. Howden, „The Boys’ Book of Locomotives”

One of George Jackson Churchward's first designs for the GWR was a 2-6-2T passenger tank locomotive intended for suburban service. The Prototype number 99 was completed in 1903 and formed the basis of a family of similar locomotives built through 1949, nicknamed “Large Prairies” to distinguish them from the GWR's smaller 2-6-2Ts. As with most of the locomotives in this family, the No. 99's driving wheel diameter was five feet and eight inches. This allowed sufficiently high speeds for their area of operation, but at the same time adequate traction was still available. With outside cylinders, it was modern for a British locomotive of the time, which made a larger cylinder diameter possible using the GWR's loading gauge

The first series consisted of 39 examples, all of which were delivered in 1903. These differed from the prototype in that the tops of the water tanks were sloped slightly at the front to improve visibility. After getting the numbers 3111 to 3149, the number 99 was redrawn to 3100 in 1912. As a result, the locomotives were now known as class 3100, but this changed again in 1927 with the renumbering to the numbers 5100 and 5111 to 5149.

Schematic drawing
Schematic drawing
Locomotive Magazine, August 1905

In 1938 and 1939 the prototype and nine other production examples were rebuilt with two inch smaller wheels and a new boiler with 225 instead of 200 psi pressure. These engines achieved a slightly higher tractive effort and were listed as Class 8100. However, since the increase in tractive power was negligible during operation, no further engines were converted and the class 8100 was used together with the class 5100.

Variant5100rebuilt 8100
Axle config2-6-2T (Prairie) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length41 ft 0 in
Wheelbase31 ft 9 in
Fixed wheelbase14 ft 9 in
Service weight169,120 lbs
Adhesive weight121,072 lbs
Axle load39,424 lbs
Water capacity2,402 us gal
Fuel capacity10,080 lbs (coal)
Power sourcesteam
Estimated power1,025 hp (764 kW)1,150 hp (858 kW)
Optimal speed25 mph
Starting effort25,669 lbf29,752 lbf
Power Plant
Driver diameter68 in66 in
Boiler pressure200 psi225 psi
Expansion typesimple
Cylinderstwo, 18 1/2 x 30 in
Grate area20.4 sq ft
Firebox area121.8 sq ft
Tube heating area1,145.2 sq ft
Evaporative heating area1,267 sq ft
Superheater area82 sq ft
Total heating area1,349 sq ft
Calculated Values
steam locomotive
tank locomotive
George Jackson Churchward
last changed: 02/2022

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