Why did some 1st generation DMU carry carriage numbers in the 79XXX range?

[Xenophon PCDGS]

This is something that I pondered upon in past times but never followed up on.

 

[StephenHunter]

They didn't generally have unit numbers, not until the 1980s.

 

[Xenophon PCDGS]

They didn't generally have unit numbers, not until the 1980s.

Perhaps I have phrased my query poorly. I meant the numbers on the actual carriages, not their set numbers. My apologies. I have now edited the thread title.

 

[pdeaves]

Perhaps I have phrased my query poorly. I meant the numbers on the actual carriages, not their set numbers. My apologies. I have now edited the thread title.

... as opposed to the 5XXXX series generally used.

 

[Magdalia]

The first BR DMUs were numbered in the 79xxx series. These included Derby lightweight and Metro Cammell DMUs with yellow diamond couplings and Swindon Inter City DMUs with white circle couplings, plus a few 4 wheeled rail buses.

The 5xxxx number series came a bit later, nearly all of these were blue square coupling, which effectively became the standard. But the 5xxxx series also included a few more Swindon Inter City DMUs with white circle couplings, and the Lea Valley suburban DMUs which were orange star coupling. It does seem odd that these exceptions were not also numbered in the 79xxx series.

 

[Taunton]

There was incompatible control logic between the Yellow Diamond and the Blue Square cars, so one the numbers were one way of identifying different types. Possibly the coloured symbols came a bit later. The Swindon 79xxx Inter-City White Circle cars were an adaptation of the Yellow Diamond system, while the later Swindon 5xxxx White Circle cars were an adaptation of the Blue Square system. Although coded the same, initially one was incompatible with the other, but it wasn't too different, and the Scottish Region later made modifications so they both worked the same and there was some intermixing.

The Yellow Diamond cars had mu jumpers high up, under the windscreen, whereas the Blue Square cars had them down at the bufferbeam, so you couldn't get them connected anyway. There were also some starting sequence wiring differences to overcome batteries being flattened too easily when trying to start Yellow Diamond cars.

For actual numbers, it became apparent after starting that the 79xxx sequence would not be sufficient for the large number of cars now to be ordered, so a complete break to a new series made sense when the control systems changed.

 

[Helvellyn]

The first BR DMUs were numbered in the 79xxx series. These included Derby lightweight and Metro Cammell DMUs with yellow diamond couplings and Swindon Inter City DMUs with white circle couplings, plus a few 4 wheeled rail buses.

The 5xxxx number series came a bit later, nearly all of these were blue square coupling, which effectively became the standard. But the 5xxxx series also included a few more Swindon Inter City DMUs with white circle couplings, and the Lea Valley suburban DMUs which were orange star coupling. It does seem odd that these exceptions were not also numbered in the 79xxx series.

I believe the 79xxx was allocated for Diesel Mechanical Multiple Units when BR standardised the numbering range. Diesel Electrical Mechanical Units had their vehicles numbered in the 60xxx series; the 61xxx-67xxx was for EMU motor vehicles; 68xxx for EMU parcel vehicles; 69xxx for EMU catering vehicles; 70xxx-74xxx series for EMU trailer vehicles; and 75xxx-78xxx series for EMU Driving Trailer vehicles.

The 5xxxx series was originally used by BR for suburban loco hauled coached stock; As BR decided to order a lot more DMUs these vehicles were all renumbered in gaps in the 4xxxx series, with the 5xxxx series then re-used for DMU vehicles (50xxx/51xxx for Driving Motor vehicles; 55xxx for Single Motor cars; 56xxx for Driving Trailers; and 59xxx for Trailers).