number are built for non-revenue earning use. Initially stock inherited from the 'Big Four' companies was given regional prefixes (e.g. DE, DM, DS and DW) indicating their origin, and adapting existing number series. From about 1951, British Railways started to use new numbering series for additions to departmental and internal user stock on a regional basis, as follows: No prefix letters were used for internal user stock. Departmental wagons (and some passenger-rated non-passenger carrying coaching stock) often kept their original revenue earning stock number, but with the addition of the "D" prefix. On some occasions, passenger coaches that had been converted for use as wagons then entered departmental stock retaining their wagon number (most notably brake vans in the 963xxx series that were formerly passenger brake vehicles). By the end of the 1960s, British Rail-built carriages were entering departmental stock and being allocated one of the regional prefixes according to their location, but from 1967 the "DB" prefix was introduced for all additions to departmental stock regardless of origin. This prefix was prefixed with a letter to indicate the use of the vehicle, as follows: The principal numbering series for carriages with the "DB" prefix have been 975xxx, then 977xxx. The latest series to be used is 971xxx. Carriages built new into departmental stock have usually been numbered in the 999xxx (and sometimes 998xxx) series (though this series also contains some conversions too). Also in the early days of the "DB" prefix, some departmental locomotives were numbered in the 966xxx and 968xxx series, though in recent years those locomotives that remain self-propelled have been allocated locomotive Class 97. TOPS CARKND classification system When the Total Operations Processing System was introduced by British Railways, classifications were applied to all carriages and wagons and recorded in a field called CARKND, which is now also used to refer to the classification system as a whole. The classification typically comprises three letters, the first of which indicates the broad type, as follows: The second letter gave more detailed information, different for each series. The tables below list the variations for carriages: Apart from hauled passenger carriages ('A' series), the final letter indicated the braking arrangements. Nowadays almost all stock is air-braked, but when TOPS was introduced there was much greater variety, which made marshalling trains more complicated and this information essential. The letters were: For hauled passenger carriages ('A' series), the suffix denoting the