Some locomotives retain just one number throughout their entire career, whether it be several years or several decades, whilst others have had more than one number allocated to them. The current TOPS (Total Operations Processing System) numbering sequences, whereby the fleet was divided by Class and sub Class, being introduced in the early 1970s superseding the D and E prefix numbering schemes from the modernisation plan of the mid 1950s which in turn had replaced an earlier BR scheme introduced in the late 1940s.

The data tables within the site are set out in sections based on the original numbering sequences used. Locomotives introduced with the modernisation plan D series are listed together, then the E series followed by those not allocated a number in the modernisation plan numbering schemes. Lastly those locomotives introduced in the TOPS era are listed. All subsequent renumbering that has taken place is listed in chronological order reading across the page from left to right.

A Class referral section is provided to work backwards from the current Class allocation to the earlier number ranges and reference to locomotives that have been reclassified during their careers may be found in more than one section.