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London & North Eastern Gresley class A4
Great Britain | 1935 | 35 produced
No. 4468 “Mallard”
No. 4468 “Mallard”
collection Taylor Rush

Gresley developed the Class A4 locomotive for the “Silver Jubilee” express train, which consisted of specially developed articulated cars. It was based on his A3 and was streamlined. Like this, it had three cylinders, but a higher boiler pressure and further optimizations within the boiler. What was striking was that the axles were in conventional friction bearings, as the initially planned German roller bearings could not be used for political reasons.

“Union of South Africa” with streamlining cut out for easier maintenance
“Union of South Africa” with streamlining cut out for easier maintenance
Andy Hawkins

The “Mallard” achieved particular fame because on July 3, 1938 it reached the still official world record for steam locomotives at 125 mph (201.2 km/h), which is controversial. It was reached on a slight down gradient and resulted in damage to a connecting rod bearing, while higher speeds on the flat were unofficially recorded in other countries. Today there are still six A4s, three of which are still operational. Among the ones that still exist today is the “Mallard”, which is not operational.

General
Built1935-1938
ManufacturerDoncaster
Axle config4-6-2 (Pacific) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length71 ft 0 3/8 in
Wheelbase35 ft 9 in
Fixed wheelbase14 ft 6 in
Service weight230,720 lbs
Adhesive weight148,000 lbs
Total weight374,415 lbs
Axle load49,280 lbs
Water capacity6,005 us gal
Fuel capacity17,920 lbs (coal)
Power
Power sourcesteam
Estimated power2,800 hp (2,088 kW)
Optimal speed50 mph
Top speed90 mph
Starting effort35,455 lbf
Power Plant
Driver diameter80 in
Boiler pressure250 psi
Expansion typesimple
Cylindersthree, 18 1/2 x 26 in
Boiler
Grate area41.2 sq ft
Firebox area231.2 sq ft
Tube heating area2,344.8 sq ft
Evaporative heating area2,576 sq ft
Superheater area749 sq ft
Total heating area3,325 sq ft
Calculated Values
steam locomotive
express
streamline
Herbert Nigel Gresley
last changed: 10/2023
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