The GCR and GNoSR operated a strict policy of not permitting any gaps in the number series, nominally for accounting purposes, but this was not followed on other lines.
The first solution, applied from September 1923 was to add a letter suffix to numbers that indicated the original owning company.
This involved adding a multiple of thousands to each locomotive number, except: Other duplicate list locomotives were renumbered in the same manner as those in the capital list; i.e. with the prescribed multiple of thousands added, but retaining their prefix or suffix as appropriate.
Departmental (non-revenue earning) locomotives continued to be numbered in their own, locally-applied, series.
The reason was that new plans had been drawn up to carry out a wholesale renumbering of engines, as set out below.
The renumbering scheme was based on the locomotives that were in stock as of 4 July 1943 but was not issued until December that year.
Self-evidently the renumbering process was very complicated as the LNER was keen not to have two locomotives running in service with the same number.
Examples included: Locomotives inherited from the Lancashire, Derbyshire and East Coast Railway retained their existing classifications (lettered A to D) and the locomotive inherited from the Wrexham, Mold and Connah's Quay Railway was left unclassified.
Where locomotives were purchased from outside manufacturers, they were referred to by the lowest number applied to a class member.
From 1900, the GNR adopted a system of classification based on a locomotive's wheel arrangemenent (using the Whyte Notation), with each arrangement being represented by a letter.
There were some exceptions to this system: The three minor companies absorbed by the LNER in 1923 and 1924 were too small to need to classify their locomotive stock.
The answer, approved in September 1923, was a variation of the GNR system, using a letter to indicate each type's wheel arrangement (based on the Whyte Notation).
The letter 'P' was included in the original list, even though there were no locomotives of the 2-8-2 wheel arrangement until June 1925, because Class P1 was at an advanced stage of design – the order was placed in November 1923.
New build and converted locomotives to LNER designs were allocated class numbers in appropriate gaps in the series.
On occasions, this led to the re-use of class numbers left vacant following withdrawal or rebuilding of its members.
During 1942, three new types were allocated the appropriate letter classification ('A', 'B' and 'D') but without any suffix number; these were prototypes for what were planned to be the new standard LNER classes.
However, this approach was not maintained, and the production types were allocated standard class numbers.
A number of classes inherited by the LNER were not formally allocated a place in the new system.
Most of these were one-off or departmental engines, or types slated for early withdrawal, although some of the latter survived in stock through to 1928.
Engines absorbed from the smaller companies during 1923-1924 (the CV&HR, E&WYUR and MSLR) were either withdrawn before a classification could be allocated or were fitted into appropriate gaps in the class list.
For the ex-M&GNJR engines (absorbed 1936), their original single letter classification was maintained through to 1942 when they were, at last, allocated class numbers at the end of the appropriate series.
In contrast, the ex-Metropolitan Railway engines (absorbed 1937) were allocated LNER classifications immediately.