At the beginning of the GER period, most locomotive classes were referred to by the running number of the first engine of the class; 'No. 231 Class' and so on. However, Gooch had started to identify his own new locomotives as classes 'A', 'B' and 'C'. Robert Sinclair, as his successor, decided to do likewise, but started at the other end of the alphabet. Thus, his five new designs were classes 'Z', 'Y', 'X', 'V' and 'W', in order of introduction.
S.W. Johnson adopted the system that was used for the remainder of the company's existence. If the first member of a class was built by an outside contractor (as were most new locomotives of the period) the class was known by the running number of the first engine, as above. Sinclair had initiated a system of order numbers known as the 'Appropriations List' for items of capital equipment built at Stratford Works. This series of numbers began at A1, B1, C1 and so on to Z1, and continued A2, B2 and so forth. The series was used for locomotives, tenders, carriages, wagons, road vehicles, and much else besides. From Johnson's term of office, locomotives of classes that were first constructed at Stratford took their classification from the order number of the first batch. Thus, his 0-4-2Ts were class T7. In later years the system was slightly modified, an 'R' being added to the classification in some instances to denote a significant rebuilding of a class. For example, the rebuilds of the R24 class 0 6 0 tank engines with high pressure boilers and larger tanks were reclassified R24R.
The LNER was the only one of the 'big four' companies created in 1923 to re-classify the locomotive stock that it inherited. The system was a modification of that used by the Great Northern Railway, using a letter to denote the wheel arrangement, followed by a number to identify the class. Thus, the Y14 0-6-0s of the GER became LNER class J-15.