Older locomotives would then be either withdrawn or renumbered into the 'duplicate list' series used for those no longer in capital stock but which not yet completely life expired.
When the London, Tilbury and Southend Railway (LTSR) was absorbed by the MR in 1912, its locomotives were renumbered into this scheme.
The scheme dealt with two key problems faced by the new company: These advantages more than overcame the disadvantage of the effort involved in renumbering almost every locomotive and giving them a number that usually bore no relation to its pre-Grouping identity, except for the Midland Railway locomotives that had been renumbered along similar lines in 1907 and mostly retained their numbers.
In 1932, as older locomotives had been withdrawn and new standard LMS designs were becoming more common, it was decided that modifications should be made to the numbering system.
Diesel shunters, which started to appear from the early 1930s onwards, were numbered in the same series as steam locomotives.
The prototype mainline diesel locomotives, the first of which was introduced at the end of 1947 just prior to Nationalisation were given the 'significant numbers' 10000 and 10001.
The principal downside with this method of classification was that it did not distinguish between particular classes of locomotive, so many very different types would have been classified '4F' for instance.
From 1953 until 1957 (but remaining painted on locomotives until much later), the suffixes 'FA' and 'FB' were sometimes used to distinguish between freight locos with different load limits due to their braking characteristics on unfitted goods trains.