TOPS Class allocation for 1st Gen DMUs

[billy_anorak59]

I’m familiar with the TOPS class allocation rationale for pre-TOPS locomotives whereby the TOPS class number allocated roughly increased with the power output of the locomotive, but does anyone know the rationale regarding the class allocation numbers for 1st generation DMUs?

 

It would seem that there is a broad demarcation (this from Wiki), but not down to the specifics of why one class got a particular number over another:

 

Class 100 to 114    ‘Low Density' passenger units (i.e. few doors per carriage) - mostly short (57'0") frame

Class 115 to 127    Mix of 'High Density' (i.e. doors to every seating bay) and 'Cross-Country' (long distance) passenger units - long (63'6") frame

Class 128 to 131    Parcels units

 

I’ve looked, but I can’t find anything more on this.

For example, why was Class 100 allocated to the Gloucester units, and Class 109 to the Wickhams? Both introduced 1957-58, both with a power output of 2x150bhp. Both ‘Low-density’.

 

There must be a reason, probably obvious, but I can’t work it out. Area of operation based, or was it actually pot luck?

There’s probably a similar story for the pre-TOPS EMUs too, but with the added complication of pre-nationalisation units.

 

No reason for knowing BTW, I’m just intrigued - the question occured to me while browing the latest 'Diesel Dawn' bookazine on DMUs...

I’m sure someone here will have the answer!

[russ p]

1 hour ago, billy_anorak59 said:

I’m familiar with the TOPS class allocation rationale for pre-TOPS locomotives whereby the TOPS class number allocated roughly increased with the power output of the locomotive, but does anyone know the rationale regarding the class allocation numbers for 1st generation DMUs?

 

It would seem that there is a broad demarcation (this from Wiki), but not down to the specifics of why one class got a particular number over another:

 

Class 100 to 114    ‘Low Density' passenger units (i.e. few doors per carriage) - mostly short (57'0") frame

Class 115 to 127    Mix of 'High Density' (i.e. doors to every seating bay) and 'Cross-Country' (long distance) passenger units - long (63'6") frame

Class 128 to 131    Parcels units

 

I’ve looked, but I can’t find anything more on this.

For example, why was Class 100 allocated to the Gloucester units, and Class 109 to the Wickhams? Both introduced 1957-58, both with a power output of 2x150bhp. Both ‘Low-density’.

 

There must be a reason, probably obvious, but I can’t work it out. Area of operation based, or was it actually pot luck?

There’s probably a similar story for the pre-TOPS EMUs too, but with the added complication of pre-nationalisation units.

 

No reason for knowing BTW, I’m just intrigued - the question occured to me while browing the latest 'Diesel Dawn' bookazine on DMUs...

I’m sure someone here will have the answer!

 

Must have been done later than the loco classes as Derby lightweights, 79xxx met cams were not included even though they survived until 1969 and none of the railbuses were included some of which lasted until 1968

[Legend]

Swindon units , which were mainly low density cross country units were 120,123,124 and 126 (my favourites) , while single unit high density were 121 and 122 . Could it be there was actually no rationale behind it and they just doled out class numbers as they came up? 

[markw]

The standard branch line units were numbered according to engine hp, 100 to 109 = 150HP, 110 and 111 = 180HP, 112 to 114 = 230HP.

There does not seem to be any logic to the Suburban, Inter-city, cross country, Trans Pennine, Single or Parcels units

[Mol_PMB]

I’m no expert but I thought they had two goes at class numbering for the DMUs, and what we are familiar with now is the second attempt. Didn’t each car type have a different class number originally?

[billy_anorak59]

6 minutes ago, Mol_PMB said:

’m no expert but I thought they had two goes at class numbering for the DMUs, and what we are familiar with now is the second attempt. Didn’t each car type have a different class number originally?

Ooh! Don't know, not heard of that - interesting.

[Simon Lee]

32 minutes ago, billy_anorak59 said:

Ooh! Don't know, not heard of that - interesting.

Have a look at the railcar site www.railcar.co.uk that explains the original class numbering of each type of car

[markw]

Just realised 125 to 127 are non standard coupling codes, which explains why the Suburban units aren't numbered consecutively.

[Mol_PMB]

24 minutes ago, Simon Lee said:

Have a look at the railcar site www.railcar.co.uk that explains the original class numbering of each type of car

Indeed, thanks for the tip. This is the relevant page:

Railcar.co.uk/numbering/tops

 

[Jeremy Cumberland]

I clearly recall the sub-classes, and the different classes for trailers and driving trailers in the Ian Allen books of the mid-1970s (although I couldn't tell you what any of the trailer classes or sub-classes were), but I have no recollection at all of classes 102 and 106 for different-engined variants. Did these disappear before the general reorganisation?