The DRG locomotive classification system was developed by the German Imperial Railway Company or Deutsche Reichsbahn-Gesellschaft (DRG), which was formed in 1924 following the merger of the German state railways (Länderbahnen) in 1920.
Similar systems for electric and internal combustion engined locomotives and railbuses were not developed until the 1930s, as the numbers of these types of motive power continued to rise and the previous schemes proved no longer adequate.
Locomotives taken over from the state railways on the foundation of the DRG were grouped into overall classes according to wheel arrangement and role, with each sub-class (Gattung, the equivalent of a former state railway 'class') being allocated its own range of serial numbers within its class.
A system of 99 class numbers (Baureihennummer, originally called Stammnummer) was established which were grouped as follows: The lower class numbers in each of the first five groups were reserved for the new standard locomotives Einheitslokomotiven that were to be built.
The higher numbers in each group were given to the state railway locomotives inherited by the DRG.
Their division into state railway sub-classes was achieved by allocating sequential serial numbers, with the last two digits of the serial number being used to denote the individual vehicle and the one or two preceding digits indicating its sub-class.
They were distinguished from steam locomotives by a preceding letter E. The engines were arranged according to top speed and the number of driven and carrying axles.
If the characteristics and design were otherwise the same, locomotives with a side-rod drive were given lower class numbers than those with individually driven axles.
The term 'small locomotive' was introduced by the Deutsche Reichsbahn in 1931 for engines that were employed in shunting duties, by stations and industrial installations.
However they required clearance for working on the open line in order, for example, to be able to deliver goods wagons to junctions outside of station yards.
Within this 'job description' there were vehicles called Rangierschlepper that worked exclusively within station yards and industrial sidings.
The main letter was supplemented by two further letters describing its technical features: In addition the operating numbers were divided into power groups: Railbuses were arranged by the DRG within the spare number ranges for passenger coaches.
In 1940 new rules were issued by the RZA München for the designation of accumulator cars, however they were not renumbered until 1948 and then by the Deutsche Bundesbahn.
To distinguish between the various types, the following system was adopted: When the railbuses were taken over by the Federal Railways of Austria (BBÖ), numbers after 900 had to be reserved.
A classification system comparable to that of the electric railbuses was introduced by the Deutsche Bundesbahn in 1948.