The numbering system seems as arbitrary as for steam locos. Class 44/45/46 started from D1 and class 40 from D200, perhaps 45s were ordered first though 40s were delivered sooner. 31s which were also one of the earliest started from D5500. Reading older magazines it's sometimes confusing to work out what modern class an article is referring to though numbers are often prefixed by something like "EE type 4" or "BR sulzer type 3". Was there a system to numbering or did they just allocate a block of numbers when needed?