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Being a kettle man Im often flumoxed by the classification systems used on modern image stuff, for instance, (and this is because Ive just got one) what is a co-co? what do the numbers mean eg class 47, class 50 etc?

4,497 Posts
Dear Steve,

The Co-Co business is the diesel's equivilent of a steam loco's 4-6-0 wheel description. In this case it is a loco with two bogies, each with three axles, each axle with its own electric motor.

A B-B would be a diesel with two two-axle bogies, the axles mechanically linked to the power source - for example the Hymek diesels (D70XX, class 35)

A1A-A1A is a two-bogied loco, each bogey with a powered/unpowered/powered arrangement of axles - Class 31 locos (D55XX) are one of the best examples.

The smaller diesel shunters used the steam notation, such as 0-4-0 or 0-6-0.

The Class numbers were introduced when the computerised system for recording wagons, coaches and locos was started up. There was not a great deal of logic to the allocation of the class numbers.

One of the best sources of comparisons between the old and new numbering systems is in the 'Preserved Locomotives of British Railways' by Platform5 Publishing.

John Webb
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