I have heard many times that the "TOPS" class numbers were necessary as TOPS couldn't cope with the previous numbering system.
From previous discussions on this, the true situation appears to be that the class numbers were thought up before TOPS, but TOPS introduction gave a justification for renumbering locomotives to include the new class numbers.
Of course some locomotives ran into the 70's without TOPS numbers and it seems that it happily coped with this.
Various statements are made about what TOPS can and cannot cope with (such as numbering starts at 001 because it couldn't handle 000) but I think it turns out that the locomotive number is just effectively a free text field and anything can go in it, and numbers start at 001 because non mathematicians generally don't like starting to count things from zero.
It's interesting that TOPS was being used for locos with pre-TOPS numbers, such as Westerns. A loco enquiry in TOPS seems to be happy with any number of digits up to five. For example, if you try to do an enquiry on a loco number such as 3 or 23 it will tell you there is no record of the engine's existence, but if you enter a six-digit number it returns ?????????? indicating it is not programmed to accept that number of digits in a loco number.
I'm pretty sure that TOPS stores locomotive numbers in a different database (the "locomotive file"?) to coaching stock and wagons.
Yes, loco enquiries (E3) are different to vehicle enquiries (J6) which can include wagons, coaches and multiple unit vehicles. It's the same when entering consist information, on freight trains the loco number goes on an "06 line" while other vehicles go on "02 lines" (with 06 or 02 being a line identifier which tells TOPS what kind of input you are making). For example, if multiple unit stock is being delivered in a freight train, the wagons and EMU vehicle numbers can all be entered using "02 lines."
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Notes on wagon numbering:
With wagon numbers in TOPS, an owner prefix is considered part of the number. For example
VTG 57231 is a TTA wagon, while 57231 is part of Class 150/2 unit
150231
It appears that TOPS didn't result in much renumbering of the BR wagon fleet; for example the "B series" 6-digit numbers for vacuum braked wagons were in place well before TOPS. One minor change (perhaps not directly related to TOPS but occurring about the same time) was the introduction of another 6-digit number series, without any prefix, for wagons built with air brakes. The MGR wagons originally had a B prefix, with the first one being B350000, but the B was later dropped to put them in the new air-braked number series.
A bigger change was the introduction of the 3-letter TOPS
CARKNDs, such as HAA, to distinguish wagon types. Before becoming known as HAAs, the wagon type was displayed as "HOP 32 AB" on the side of the wagons (32 ton capacity hopper, air braked).
Pre-nationalisation wagons had their regional prefixes (S, M, W, E) to prevent them conflicting with other number series. The biggest numbering change for wagons was in the private owner fleet. Previously, it appears the wagon owners would use their own numbering schemes. In a David Larkin book on "non-pool freight stock" it's surprising to see a photo of a modern-looking BP tank wagon (later TTA) in 1968 numbered 32. The specific wagon was owned by STS and on hire to BP Chemicals, and referred to in the text as "STS 32" to distinguish it from any number of other private wagons that might also have been numbered 32. The idea of an owner prefix was implemented in TOPS numbering for private owner wagons, and they were also renumbered with a four or five digit number (such as the example of VTG 57231 earlier).
The HTV and MCO wagons were marshalled into sets with the 4 digit set number stencilled on the side of each wagon. The MCO sets ran with a stencilled brake van at each end; gradually the vans gave way to fitted heads, about 3-4 MCV displaced the same numer of MCO at each end as well as the van. I am pretty sure those wagon set numbers were computer data related as they had no 8 or 9 in the number range from what I saw which suggests they were octal numbers.
Those were TOPS pool numbers, with some listed here:
https://www.rmweb.co.uk/forums/topic/162893-tops-wagon-pool-identities/
In some cases a pool will effectively constitute a set, with a few maintenance spares.
This is a good example of an MSV displaying a prominent pool number for stone traffic:
https://paulbartlett.zenfolio.com/ironoretipplermsv/h2163d027#h2163d027
And here is an HTV displaying pool 7509:
https://paulbartlett.zenfolio.com/brrebuilthtohtv
I looked for photos of these wagon types in Mendip stone traffic taken around 1972 and managed to find a couple.
Here at Merehead in May 1972, HTVs display their pre-TOPS type code of "HOP 24 VB" along with weight information and their B series numbers:
http://www.hatspics.co.uk/photo_expanded.php?id=7205009
There are some interesting 1970s photos on that website, with an index by year here:
http://www.hatspics.co.uk/type_years.php?type=British Railways
Here in a rather nice 1972 sequence of photos at Merstham (with a warship unloading HTVs at a temporary terminal for M25 construction) the wagons are again in a pre-TOPS state displaying "HOP 24 VB," with "HOUSE COAL CONCENTRATION" revealing their former occupation:
https://www.flickr.com/photos/26292271@N05/31866811214/in/photostream/
In a "Foster Yeoman: The Rail Story" booklet (probably from an open day at Merehead) there's a Feb 1972 photo of D1662 with MCVs at Merehead, which again are in their pre-TOPS state with lettering displaying STONE, no pool code, with weight info and the B series number.
This was all taking place before I was born, but I can't help thinking the Mendip stone traffic must have been a great sight behind a Western or Warship!
821 at Westbury with stone empties:
https://www.flickr.com/photos/52554553@N06/10421811905/
D1005 with stone empties:
https://www.flickr.com/photos/52554553@N06/10421711036/