What's In A Number?

Over the next few weeks I thought that it might be fun to take a more detailed look at each class of diesel locomotive, as allocated under the TOPS classification.

So, beginning with Class 01, I intend to give a brief history of each loco class, its technical specification and numbers built etc.

However, before I do that I thought it would be a good idea to take a brief look back at the history of diesel locomotive numbering since it can be a little confusing. 

It was in 1957 that the then British Transport Commission announced a new numbering system for diesel locomotives and this involved the use of the prefix letter "D" (for Diesel) followed by a number. This number would not only identify the locomotive but would also indicate its power range.

At the same time the power of main-line locomotives was also to be indicated by Type numbers and these would be in the following ranges:

Type 1:   Up to 1,000 bhp

Type 2:   1,000 - 1,499 bhp

Type 3:   1,500 - 1,999 bhp

Type 4:   2,000 - 2,999 bhp

Type 5:   3,000 bhp and above

The table below shows the relationship between the number range of the locomotive, its type and engine power:  

Subsequent to this, however, new batches of locos were delivered that were numbered outside of the original groups for its type with the result being that it soon became difficult to ascertain the power of a locomotive solely by its number.

With the withdrawal of standard gauge steam locos in 1968, the "D" prefix was discontinued and then, from 1973, BR applied a new numbering system to both their diesel locomotives and multiple units. 

This classification was known as the Total Operations Processing System or TOPS for short and the numbers were in the format 'xxx zzz' where 'xxx' is the class number and 'zzz' is the unique locomotive identifier, for example 33 025 and 45 053

The table below, therefore, shows the relationship between this class numbering system, the type number, the power range and the original "D" numbering system.

Under the TOPS scheme, all locomotives (with a few exceptions) had a unique number in the form xx001 with the majority of locomotives retaining the final two digits of their "D" numbering. As such, D411 became 50 011 and so on.

One exception to this rule was with the first loco in each class, which had usually been numbered "Dxx00". The problem being that TOPS could not cope with numbers ending in 000 and that is the reason why there has never been a 37 000 or a 47 000. In these instances the locomotive concerned would either be renumbered and moved to the end of the class, hence "D400" became 50 050, or it would take the number of a loco that had been withdrawn from service.

Of course this is a very simplistic look at diesel numbering and I appreciate that I have not covered multiple units nor locomotive sub classes but I promise to cover these in a separate posting.