If you ignore TOPS and look at all the BR diesel numbering schemes then some of the early shunter designs become nice and confusing.
For instance what became class 10 were numbered starting 13137, while what became class 11 started at 12033
Even when they later got D numbers they weren't numbered in class specific blocks
Almost all of them were in class-specific blocks, although some of them overflowed and needed a second block (e.g class 25, continued after 5299 at 7500). There were exceptions where TOPS made a distinction that had not been made before.
The standard shunters were numbered in one block starting at 13000, later re-numbered in the D3000 block. It was only with TOPS that they were allocated to different classes according to the engine and/or gearing fitted, hence classes 08,09 and 10 all coming from the D3000 to D4192 range. Likewise classes 22/29, 30/31, 42/43, 47/48. Also, Classes 24/25, 26/27, 44-46 were each numbered in one block, rather than resuming at the next round hundred (e.g the first Class 46 was D138).
12033 was the first of the LMS-design shunters with axle-hung motors, which were classified class 11 - 12000-12032 were the older jackshaft type.
There was some logic to it:
Large main line diesels in the 10xxx series (there were only six, of three different types: 10000 and 10001; 10100; and 10201,10202,10203.
Diesel mechanical shunting engines in the 11xxx series. These had several blocks (starting at 11000, 11100, 11500, 11700) depending on transmission and wheel arrangement. They became the D2xxx series under the 1957 scheme, the 11100 series, which had got very tangled, being split into D2000 (later class 03) D2200 (class04), D2400, D2500, and D2550 (class 05), the 115xxs becoming the D2950 class (later Class 01) and the 117xx becoming D27xx. Vacant spaces (from D2410, D2510, D2850 and D2900) were later used by new classes, the D2410s and D2850s surviving long enough to become Classes 06 and 02 respectively.
Diesel-electric shunting engines to LMS designs (three classes) were numbered by BR in the 12xxx series, with no gaps. LMS numbers 7069 - 7131:those still extant at nationalisation became 12000-12044 under BR and production continued under BR to 12133. Pre-nationalisation designs did not get D-prefix numbers, so they retained their 12xxx numbers until withdrawal, although the third class (12033 onwards) became TOPS Class 11,being withdrawn before getting individual TOPs numbers.
The BR development of the 12033 class was, of course, the standard shunter we now know as Class 08, originally numbered in the 13xxx series (up to 13366), and later in the D3xxx series. A few batches were built with non-standard gearing, and others with non-standard Blackstone or Crossley engines. These were numbered in the 13xxx/ D3xxx series mixed with those to the standard spec, but under TOPs two of them became classes 09 and 10 (the Crossley type (13117-26) had all gone by then)
Variants of the standard LMS shunter were built for the other members of the "Big Four", numbered by BR in the 150xx (LNER), 151xx (GWR) and 152xx (SR) series. A further batch of the SR type were built after nationalisation, numbered from 15211 to separate them from the pre-war version (15201-15203), and later became Class 12
The two gas turbine locos were numbered 18000 and 18100
Pre-nationalisation electric types
SR CC1 - CC3 became 20001-20003
LNER EM1 26xxx (26000 was LNER 6701)
LNER EM2 27xxx
(these became classes 70,76,77, although only class 76 received individual numbers under TOPS)
Former NER locos: 265xx (original NER numbers 1-13, LNER numbers 6480/81, 6490-99, 6999: these were respectively the two 1903 shunting engines, the ten Newport-Shildon locos, and the 1922 prototype for the NER's planned York-Newcastle electrification: Nos 3-12 had been in store since 1935 and No13 ever since it was built.
Most of the gaps in the TOPS sequence can be explained in terms of what was still around in the mid-60s
Classes 18,19. There were actually three different engine makes fitted to Class 17. Probably the intention was to give each a different class number.
Classes 32, 34. The push-pull and Hastings gauge variants of Class 33
Class 36. One of the LMS twins, No10001, was still in service in 1966, and neither was actually scrapped until 1968.
Class 38, 39. There were a number of variations in specifications during the production run of Class 37 - maybe the intention was to divide this class like the "Peaks" were?
Classes 49, 51, 53, 54. With the first 20 ("generators"), and various different engine ratings, there was plenty of scope for the Brush Type 4s. Asit was, only the V-16 engined version got its own classification. (The use of Class 53 for the "Falcon" came later)
Class 72 The first six class 73s had several differences from the rest of the class.
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