The change in class designation to A3 reflected the fitting to the same chassis of a higher pressure boiler with a greater superheating surface and a small reduction in cylinder diameter, leading to an increase in locomotive weight.

The new Pacific locomotives were built at the Doncaster "Plant" in 1922 to the design of Nigel Gresley, who had become Chief mechanical engineer of the GNR in 1911.

The intention was to produce an engine able to handle, without assistance, mainline express services that were reaching the limits of the capacity of the Ivatt large-boilered Atlantics.

However, Gresley's Pacifics had been designed to work within the bounds of the Great Northern Railway, meaning maximum distances of less than 200 miles (322 km).

In the following months, the two railway companies ran comparative exchange trials between the two types from which the Great Western emerged triumphant with 4079 Pendennis Castle.

Changes to the valve gear included increased lap and longer travel, in accordance with Great Western practice; this allowed fuller exploitation of the expansive properties of steam and reduced back pressure from the exhaust, transforming performance and economy; the economies in coal and water consumption achieved were such that the 180 psi Pacifics could undertake long-distance non-stop runs that had previously been impossible.

The presence of the larger superheater could be recognised from the square covers on each side of the smokebox, a feature that the locomotives retained throughout the rest of their existence.

[1] This locomotive appeared in August 1928 with 220 psi (1.52 MPa) boiler, 19-inch (483 mm) cylinders, increased superheat, long-travel valves, improved lubrication and modified weight distribution.

[17] Another new development was the changeover from right- to left-hand drive, less convenient for a right-handed fireman, but more so for sighting signals, resulting in the modification of all earlier locomotives.

The last nine A3 Pacifics were constructed with the device in 1935, and it became a standard fitting on all LNER large, wide-firebox boilers that were applied to new locomotives until 1949, except for a short period while Edward Thompson was CME.

[23] A3s 2747 Coronach and 2751 Humorist were subjected to smoke deflection trials following an accident on the London, Midland and Scottish Railway (LMS) due to poor visibility; this included the modification of the upper smokebox area surrounding the chimney.

The next stage, at least with 2751, was to cut off the top part of the wrapper, but retaining the sloping plate that directed air flow upwards, and therefore lifting the smoke above the locomotive.

The original chimney was replaced by a double stove-pipe variety, and miniature deflector plates were added on each side, angled to concentrate the air flow when the locomotive was on the move.

The original A1s were coupled to a traditional Great Northern type of tender with coal rails of a design that can be traced back to Stirling days.

[28] This was largely due to a regression from the earlier 3-cylinder 2-6-0 design, which was the first to have the standard Gresley conjugated motion combined with long valve travel.

[29] However, practical problems were experienced with components quickly suffering from premature wear, especially in the main bearing of the large 2:1 lever which had not yet been fitted with the very necessary ball race; excessive 'play' led to so much over-travel of the middle valve, that it began to hit the end-covers.

In order to prevent this, when applying the gear to the Pacifics, Gresley fell back on the expedient of shortening valve travel even though that choked the exhaust at speed, was responsible for the heavy coal consumption, and negated most of the advantages gained by the locomotive's revolutionary design.

[28] However, by incorporating the Great Western-inspired valve modifications, the economies in coal and water consumption achieved were such that the 180 psi Pacifics could undertake long-distance non-stop runs that were previously impossible.

The first and most spectacular outcome occurred in 1928, when the Pacifics were called upon daily to work the Flying Scotsman train non-stop over the 392 miles (631 km) between London and Edinburgh.

[25] There were earlier claims to this speed, notably by the Great Western locomotive 3440 City of Truro, but this 1933 run is generally considered to be the first reliably recorded instance.

On a later trial run to Newcastle upon Tyne and back in 1935, A3 number 2750 Papyrus reached 108 miles per hour (174 km/h) hauling 217 long tons (220 t; 243 short tons) at the same spot, maintaining a speed above 100 mph (161 km/h) for 12.5 consecutive miles (20.1 km), the world record for a non-streamlined locomotive, shared with a French Chapelon Pacific.

Following Gresley's sudden death in 1941 Edward Thompson took over and following the end of hostilities in 1945, Thompson made a request to his works staff that a locomotive be selected for rebuild, the first of Gresley's A1 Pacifics, Great Northern was selected and became the LNER Class A1/1 specification with divided drive and separate valve gear for the inside cylinder.

Various experiments were tried over the years to cure this chronic ailment, and it was only towards the end of the steam era that a real solution was found in Great Western methods of lubrication and manufacture for the big-end bearing.

[31] In spite of all this and the introduction of more recent Pacifics, in the middle of the 1950s Gresley types continued to have a quasi-monopoly of East Coast Main Line express passenger services, and as the Sixties approached they went through yet another series of improvements comparable to those of the 1920s.

The most significant of these was the fitting of the French double Kylchap exhaust system, which was entirely due to the persistence from 1956 of P. N. Townend, Assistant District Motive Power Superintendent at King's Cross locomotive shed.

[34] The Kylchap arrangement was already being universally applied to the A4 streamlined Pacifics, though with the non-streamlined A3 locomotives, the soft exhaust would cause the smoke and steam to drift into the driver's forward vision.

After overhaul, Scotsman worked a number of railtours, including a non-stop London–Edinburgh run in 1968, the final year of steam traction on British Railways.