To some railway companies, notably the London & North Western, or the North Eastern, the numbers carried by the locomotives had an almost religious significance. It was an anathema to have a blank number in the list, and when an engine was withdrawn from service, the vacant number was re-used by the next new locomotive to be added to stock. This quickly lead to a situation in which the engines of any particular class would be scattered all over the locomotive list, and it was thus impossible to tell to which class a particular engine belonged by reference to its running number alone. The Eastern Counties Railway, in its earliest days, was one of these companies, and its engines were also frequently renumbered. This appears to have been done with the intention that the newest locomotives should carry the lowest numbers. However, by the time that the GER was formed, a more relaxed attitude had been adopted, and when engines were withdrawn, their numbers remained blank until significant blocks of consecutive numbers were vacant, and could then be re-used by new locomotives.
To speed up the process, the ECR and GER operated a 'duplicate list' into which older locomotives were renumbered to clear out 'stragglers', and thereby clear number blocks for re-use. On some railways, the 'duplicate list' was strictly an accountancy measure, being used for locomotives whose 'book value' had depreciated to zero, but which were nevertheless still fit for a few year's further service. However, the GER used the 'duplicate list' as a convenience. Having said this, the way in which duplicate list locomotives were identified was initially hap-hazard. The most common method in the earlier period was to add a zero suffix to the engine number so that, say, No. 143 was 'duplicated' as No. 1430. This worked, provided that the original number was high enough that the additional zero elevated it 'out of harm's way', beyond the highest number carried by the locomotives on the 'capital list'. If the number was a low one, such as 8, then adding the zero suffix to make 80 would be of no use. In such cases, the original solution was to add an 'A' suffix, so that the engine concerned would become '8A'.
By the early GER period, the practice in the latter case was to add a zero, but as a prefix, so that No. 8 became No. 08. By the time that James Holden became Locomotive Superintendent, the use of the zero prefix was universal, and this more sensible solution lasted until the end of the GER's existence. An extreme example of how the GER used the duplicate list as a convenience is in the case of the Worsdell G16 class compound 4-4-0s, numbers 700-709. At the beginning of 1892 they were relegated to the duplicate list as 0700-0709, so that their original numbers could be used for new T19 class 2-4-0s. However, no sooner had the 4-4-0s been duplicated than they were all rebuilt as simple-expansion engines, and they actually lasted longer on the duplicate list than they had on the capital list.
As regards the main 'capital list' numbering, when Robert Sinclair took charge, the locomotive stock of the ECR was around 300 engines, and he resolved that the numbers 300 and above should be used for goods engines, and thus his 'Y' class 2 4 0s were consecutively numbered 307-416. This policy was continued under Johnson and Adams, and the numbers 417-541 were accordingly occupied by the former's 0-6-0s and the latter's 2-6-0s. By the Bromley period, the locomotive stock had grown to the extent that new 'virgin' numbers in the 500s and 600s had to be used for passenger engines as well.
Under James Holden, numbers above 800 were reserved for new goods engines, and the series 800-945 were all occupied by Y14 0-6-0s. When he re-vamped the design as the N31 class in 1891, these were numbered backwards, in batches of ten from 999 until the series 946-999 was used. Other examples of this class - as well as the Y14s - re-used former goods engine numbers in the 500s and 600s.
By the 1890s, the expansion of the locomotive stock meant that number series above 1000 had to be used, and by the turn of the century the highest numbers were being allocated to the S44 class 0-4-4Ts of 1898 in the early 1100s. When the first S46 class 4-4-0 was ordered, it was initially intended to number it 1150, but then someone had the bright idea of giving it the number 1900 to celebrate the new century. The subsequent engines were then numbered backwards in batches of ten; 1890-1999, 1880-1889 and so on, as had been done with the N31 class 0-6-0s. This meant that the numbers 'in between' were now fair game, and thus the 1300 series were used for the Y65 class 2-4-2Ts of 1909, and the 1500 series for the S69 class 4-6-0s two years later.
Meanwhile, the numbers below 400 were re-used mainly for tank engines as they became vacant, usually in blocks of ten. Some effort was made to select contiguous groups of numbers for particular classes. For example, the first R24 class 0-6-0Ts were given the numbers 327-346 and 397-416 for the passenger and shunting engines respectively, thus re-using the numbers originally carried by the 'Y' class 2-4-0s. Following engines of the class filled in the gap 347-396, but then other odd groups of ten numbers had to be used for the later engines.
Going back even earlier, S.W. Johnson seems to have resolved to use his 'No. 1 Class' 2-4-0s of 1867-72 to 'mop up' individual blank numbers. In order of building, they were given the numbers 1-3, 26, 28, 29, 32, 35, 43, 44, 46, 47, 48, 49, 33, 34, 36, 42, 45, 50, 112, 113, 110, 114, 115, 31, 100, 104-106, 107, 160, 161, 173, 176, 119, 118, 5, 6, 7. By the time that the last had been delivered, they thus occupied the series 1 3/5-7, 26/8/9, 31-6, 42-50, 100/4-107, 110/2-5/8/9, 160/1, 173/6, and were by far the most untidily-numbered class. In later years several engines were renumbered, and they ended up with the number series 1-6, 27-36, 42-50, 103-108, 110/2-5/7/8 and 160/1, which was marginally neater.
Upon the creation of the LNER, the numbers of the locomotives of the constituent railway companies had to be altered so as to avoid duplication, and initially, ex-GER engines had an 'E' added as a suffix to their numbers. From 1924 the ex-GER engines had their numbers increased by the addition of 7000. Thus, GER No. 236 became LNER 7236; GER 1274 became 8274 and so on.
Although in the throes of the Second World War, in 1943 the LNER drew up a scheme to renumber the whole locomotive stock so that specific series of numbers were allocated to particular locomotive types. For example, the series 4000-5999 was reserved for six-coupled goods tender engines. The particular blocks of numbers allocated to each class were arranged with the engines numbered in order of age, with the oldest engines having the lowest numbers. Manpower shortages meant that it was not possible to actually begin implementation of the scheme until January 1946. The renumbering was carried out to a carefully worked-out plan, with specific locomotives being dealt with each Sunday, so as to avoid two locomotives carrying the same number at the same time, and the programme took just over a year to complete. However, prior to implementation of this renumbering, several temporary alterations took place on ex-GER locomotives to clear numbers in the 8300-8900 range for use by new B-1 class 4-6-0s, and for the LMSR-designed '8F' 2-8-0s being built at Doncaster for the War effort. A little later, some of the GE-built N-7 0-6-2Ts were renumbered to clear the 8000 series for new diesel-electric shunting engines, so that they could carry their allotted numbers under the 1943 scheme immediately.
Under British Railways, the ex-LNER locomotives were renumbered by adding 60,000 to their existing numbers, and thus ex-GER J-15 0-6-0 No. 4560 became BR No. 65460.