The LNER's Express Pacific designs were probably the most famous of the LNER locomotives. And of these Pacifics, the A4 is probably the most famous of all: Its distinctive streamlined shape has come to be a symbol of 1930s luxury and speed. Even today, it is an A4 which holds the world speed record for any steam locomotive in the world.

Nigel Gresley travelled on the 'Fliegende Hamburger' (a high-speed diesel electric) on a visit to Germany and was inspired by its fast average times and streamlining. From this, he calculated that a streamlined and modified A3 design would be able to haul trains of eight or nine carriages at similar speeds.

The wedge-shaped streamlining on the A4 was inspired by a Bugatti rail-car which Gresley had observed in France. As well as streamlining, it was important that the design lifted smoke up away from the cab. At first, there was a lot of difficulty in achieving this. During the wind tunnel tests, it was noticed that a thumb print had inadvertently been added to the plasticine model, just behind the chimney. The model was re-tested with the thumb print and amazingly the smoke was lifted well clear of the cab!

As well as the characteristic streamlining, a number of other things were modified from the A3 design. All of the steam passages were streamlined. The boiler pressure was increased from 220psi to 250psi. The cylinders were decreased slightly in size so that the valve diameters could be increased to 9 inches. This produced free steaming within the restrictions of the 3 cylinder design. As with the A3, Walschaerts gear was used on the outside cylinders, and Gresley's conjugated gear was used for the inside cylinder. Further refinements were added at a later date, including the Kylchap double-blastpipe exhaust.

6 A4's have been preserved and are represented in this pack. Mallard is part of the National Collection and is on display at the National Railway Museum. Sir Nigel Gresley is based at the North Yorkshire Moors Railway and is also registered for use on the mainline, along with sister locos Bittern and Union of South Africa. These three locomotives have also had air brake equipment fitted. Dominion of Canada and Dwight D. Eisenhower have been temporarily repatriated from their respective homes 'across the pond' to help celebrate the 75th anniversary of Mallard's record-breaking run in 2013.