LNER Thompson Class A1/1

:"Disambiguation: LNER Class A1/A3, LNER Peppercorn Class A1"

The London and North Eastern Railway (LNER) Thompson Class A1/1 was started in 1945 when a Class A1 (A10) went into the works for rebuilding in to the A1/1.Thompson had the intention of improving the A4 design, and drew up two modified Gresley 3-cylinder designs. These designs discarded the conjugated gear and had separate sets of Walschaerts valve gear for each cylinder. Due to space restrictions, the middle cylinder could not be parallel with the outside cylinders. One design had the middle cylinder forward of the outside cylinders, whilst the chosen design had the middle cylinder further back. Both designs used A4 boilersFact|date=December 2007.

A new set of frames were built for the rebuild. These were similar to A4 frames except for the wheel spacings at the front. The resulting wheelbase was 38ft 5in, even longer than the Gresley P2s.

Originally, the plan was to rebuild all of the remaining A1s that hadn't been converted to A3s but this never occurred. There were also plans for streamlining, but these were never acted uponFact|date=December 2007.

In October 1945, the order was given to construct sixteen new locomotives to the new A1/1 plan. These finally appeared during Peppercorn's tenure, by which point the A1/1 design had evolved into the Peppercorn A1 Pacific design. "Great Northern" was actually officially designated Class A1 until 1947 when it was reclassified Class A1/1 in anticipation of these new Peppercorn A1 Pacifics.

In the first few years of service, the rebuilt "Great Northern" suffered a number of teething problems. These were fixed, but it never played a prominent role on the East Coast Main Line. Although it was occasionally allocated to Kings Cross, it spent much of its career based at Doncaster or Grantham. "Great Northern" was even allocated to be the Grantham pilot for a while during the mid-1950s.

"Great Northern" was renumbered No. 113 in Thompson's 1946 renumbering scheme. It then became No. 60113 during British Railways ownership, before being withdrawn on 19 November 1962 with a badly worn middle cylinder.

"Great Northern" (the very first Gresley Pacific) was chosen for this rebuilding, rather than being selected for future preservation. There has been much debate on the reason for this choice. Some state it as bad luck, whilst others clearly blame Thompson for letting his personal feelings towards Gresley cloud his judgement. It is known that the Chief Draughtsman and a number of high up officials within the LNER unsuccessfully tried to deter Thompson from rebuilding this engine.