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
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.