You may well have noticed that on Great Western built or owned locomotives there are coloured discs on the cabsides, usually with a letter within the disc and sometimes with a white X underneath.

The colour of the disc denotes the route availability of the locomotive (by axle load) and the letter within denotes the power.

1. No coloured disc:  Unrestricted (less than 14 tons on heaviest axle)
2. Yellow disc: 14t - 16t
3. Blue disc: 16t - 17t 12cwt
4. Red disc:  17t 12cwt - 20t
5. Double Red:  22t 10cwt (The King was the only loco to carry this classification).

The letters within the discs mean the following:

• None:  Up to 14,000lb tractive effort
• A  14,000lb - 18,500lb
• B  18,500lb - 20,500lb
• C  20,500lb - 25,000lb
• D  25,000lb - 33,000lb
• E  33,000lb - 38,000lb

Anything over 38,000lb was a special, like the Kings at 40,300lb. You may think that the 47xx class would be specials but their large wheels and the fact that they had 10 wheels made them made them a Red D loco.

As for the "X"s? They were applied during the war and continued in use thereafter, they denote an engine that can be "overloaded". As shown above, the railways had set load limits for each class of engine, but it is patently obvious that some locos in a class are more powerful than others, especially after cylinder boring and tyre turning. As the railways were desperate to get oversize trains on the move, they formalised what shed foremen had probably been doing for ages and put an X under the power classification of the relevant engines.

All Modified Halls carry an X because they can cope with more load than the standard Hall, although the book tractive effort is the same. 3822 has an X because tyre turning and cylinder boring put it's calculated tractive effort over 38,000lb.

Information supplied by Neil Carr, Steam Locomotive Dept.