Designated Class 37 the 309 members of this Class when delivered were numbered in the range D6700 to D6999 with the final nine built being numbered D6600 to D6608 before the main batch.
On renumbering into the TOPS series the 308 surviving members of the Class, D6983 being an early casualty, were renumbered to 37001 to 37308 in sequence except for D6700 which became 37119 at the end of the split headcode panel build displacing D6819, the first built with a central headcode arrangement, to become 37283 in a vacant slot available due to the already scrapped D6983. D6600 to D6608 became 37300 to 37308 at the end of the series.
The only renumbering to effect the Class 37s prior to the refurbishment in the mid 1980s was the creation of a 37/3 sub series for a batch of locomotives dedicated to Scottish steel and iron traffic based on the Motherwell area. Numbered in the ranges 37310 to 37314 for the centre headcode examples and 37320 to 37326 for the split headcode variants all twelve were later returned to their former identities.

37026 became 37320 but later reverted to 37026
37037 became 37321 but later reverted to 37037
37049 became 37322 but later reverted to 37049
37088 became 37323 but later reverted to 37088
37099 became 37324 but later reverted to 37099
37108 became 37325 but later reverted to 37108
37111 became 37326 but later reverted to 37111
37137 became 37312 but later reverted to 37137
37145 became 37313 but later reverted to 37145
37152 became 37310 but later reverted to 37152
37156 became 37311 but later reverted to 37156
37190 became 37314 but later reverted to 37190

With the onset of the refurbishment of the Class 37s during the 1980s it was initially envisaged that all 308 members would be done and several new Sub Class number ranges were created to reflect the changes being incorporated during refurbishment. In its most basic form the bulk of the Class 37s were to be renumbered into three main groups; 37/4 for those refurbished and fitted with ETH equipment, 37/5 for those refurbished but not fitted with ETH equipment and 37/7 for those refurbished and ballast weighted to increase adhesion. Detail differences within the 37/5 and 37/7 Sub Classes and the need to differentiate between these led to the Class 37s on renumbering into the 37/5 and 37/7 series being renumbered both from the beginning of the number range upwards and from the end of the number range downwards with, to complicate things even further, several 37/7s being numbered outwards in both directions from the middle of the range as outlined below. One other Sub Class created during refurbishment was the 37/9 series for six locomotives re-engineered with alternative power units to standard as test beds for the then mooted Class 38 build.

In greater detail…

37/4: Refurbished Class 37s, generator replaced with an alternator, re-geared bogies and ETH equipment fitted. Renumbered from 37401 upwards to 37431. All modified from the later centre head code variant.
37/5: Refurbished Class 37s, generator replaced with an alternator and re-geared bogies. Those from the split box headcode series were renumbered from 37501 upwards to 37521 whilst those from the centre headcode built were renumbered downwards from 37699 to 37677.
37/7: Refurbished Class 37s, generator replaced with an alternator, re-geared bogies and ballast weights added. Those from the split box headcode build were renumbered from 37701 upwards to 37717 and 37799 downwards to 37796 depending on what alternator type had been fitted whilst the centre headcode variants were renumbered from 37800 upwards to 37803 or 37899 downwards to 37883 again depending on what type of alternator had been fitted.
37/9: Refurbished and re-engined Class 37s, generator replaced with an alternator, re-geared bogies and ballast weights added. Renumbered from 37901 upwards to 37906 all six conversions were from the later build centre headcode variant.

So after the refurbishment of some of the Class 37 fleet, the refurbishment of the whole fleet was abandoned part way through the program, those refurbished or re-engineered, fell into the number ranges…

37/4: 37401 to 37431
37/5: 37501 to 37521, 37677 to 37699
37/7: 37701 to 37717, 37796 to 37803, 37883 to 37899
37/9: 37901 to 37906

Also at this time several Class 37s had been receiving refurbished and re-geared bogies as per the fully refurbished Class 37s but were still standard internally. To identify these Class 37s a new 37/3 Sub Class was created with those from the split headcode build being renumbered from 37350 upwards whilst those from the centre headcode series were renumbered into the 37/3 series from 37370 upwards. Later developments and the swapping of bogies between the Class 37s saw several of these renumbered examples regaining their former 37/0 identity whilst other members of the Class on receiving re-geared bogies took other numbers in the 37/3 series with several later re-numberings falling into the range from 37349 downwards.
With the advent of the creation of the 37/3 series a handful of 37/0s were still extant numbered 373XX and it was deemed necessary to renumber these to below 373XX so 37303 37304 37306 and 37308 were renumbered to 37271 37272 37273 and 37274 respectively as these numbers were vacant due to the original 37271 37272 37273 and 37274 being refurbished and renumbered already. Whilst 37274 was later returned to its original number of 37308 and 37273 met its end numbered as such 37271 and 37272 were again later renumbered back into the 373XX number range but as 37333 and 37334 on fitment of re-geared bogies.

37002 became 37351
37008 became 37352 before reverting to 37008
37009 became 37340
37015 became 37341
37032 became 37353 before reverting to 37032
37043 became 37354 before reverting to 37043
37045 became 37355 before reverting to 37045
37049 became 37343 (2nd 37/3 number as had earlier been numbered 37322)
37053 became 37344
37068 became 37356 before reverting to 37068
37079 became 37357 before reverting to 37079
37091 became 37358
37101 became 37345 (37101 never ran in traffic numbered as 37345)
37118 became 37359
37119 became 37350
37127 became 37370
37128 became 37330
37145 became 37382 (2nd 37/3 number as had earlier been numbered 37313)
37147 became 37371
37159 became 37372
37160 became 37373
37165 became 37374 before reverting to 37165
37167 became 37383
37193 became 37375
37199 became 37376
37200 became 37377
37202 became 37331
37204 became 37378
37226 became 37379
37239 became 37332
37258 became 37384
37259 became 37380 before reverting to 37259
37284 became 37381
37285 became 37335
37303 became 37271 before renumbering to 37333
37304 became 37272 before renumbering to 37334
37306 became 37273
37308 became 37274 before reverting to 37308

One further development and an associated set of re-numberings was the creation of the 37/6 series for a batch of refurbished 37/5s from both ends of the number range, 375XX and 376XX, which were selected and modified for Eurostar use. These twelve were re-numbered into the 37601 to 37612 range leading to the situation where the 37/6 series, refurbished 37s on standard bogies, sits in the middle of the 37/5 series.

37601 renumbered from 37501 which on refurbishment had been renumbered from 37005
37602 renumbered from 37502 which on refurbishment had been renumbered from 37082
37603 renumbered from 37504 which on refurbishment had been renumbered from 37039
37604 renumbered from 37506 which on refurbishment had been renumbered from 37007
37605 renumbered from 37507 which on refurbishment had been renumbered from 37036
37606 renumbered from 37508 which on refurbishment had been renumbered from 37090
37607 renumbered from 37511 which on refurbishment had been renumbered from 37103
37608 renumbered from 37512 which on refurbishment had been renumbered from 37022
37609 renumbered from 37514 which on refurbishment had been renumbered from 37115
37610 renumbered from 37687 which on refurbishment had been renumbered from 37181
37611 renumbered from 37690 which on refurbishment had been renumbered from 37171
37612 renumbered from 37691 which on refurbishment had been renumbered from 37179

The Class 37s have been numbered in the following number ranges…

37/0: 37001 to 37308
37/3 (1): 37314 to 37326
37/3 (2): 37330 to 37384
37/4: 37401 to 37431
37/5: 37501 to 37521, 37677 to 37699
37/6: 37601 to 37612
37/7: 37701 to 37717, 37796 to 37803, 37883 to 37899
37/9: 37901 to 37906

A later development saw four Class 37s being refurbished for Network Rail and the European ERTMS project. These four Class 37s were renumbered into the departmental 97/3 series.