Southern Region units are excluded from this page, as they perpetuated the four-digit numbering series commenced by the Southern Railway - see SR Multiple Unit Numbering and Classification.

Prior to the introduction of TOPS, electric multiple unit numbers were allocated by British Rail regions in their own series. Diesel multiple units did not carry unit numbers at first, but the Scottish and Western Regions later adopted their own series for these units, whilst elsewhere individual depots allocated unit numbers, often prefixed by their two-letter TOPS depot code (e.g. TS for Tyseley).

When TOPS numbers were applied to multiple units, the unit numbers were prefixed by the class number to make the new six-digit unit numbers. These regional series have been perpetuated since TOPS because it eases the description of individual units and avoids potential confusion if there is only one unit with a particular identifier working in a certain area.