Station Area ID Codes (SIDs)

When travelling to London I use the London Underground, or, depending on my destination, the Overground. Because of this I have visited many stations, with their countless escalators, lifts, staircases, corridors and ticket halls, which make each station unique in its own way.

Another thing that they all share are small blue number plates, affixed to walls and doors throughout the station complex. Some corridors can have multiple SIDs, depending on their length.

Each plate as a single digit number above a three digit number. The top number is easy to work out as this denotes the level beneath ground, but the longer number is a lot trickier to understand.

Brief History

Following the King's Cross fire, in 1987, where 31 people died, which was also the first fatal fire on the underground, newer and more stringent regulations were put in place.

At the time there were no proper evacuation protocols in place and, as at King's Cross, when the smoke filled the subterranean labyrinth finding locations was made more difficult.

One of the new safety regulations was the introduction of Station Area ID Codes (SIDs). 

Like all station signs these SIDs have to follow regulations as to their size.

As stated before, the top number denotes the level below ground. There is a pattern to the lower number, rather than it being used arbitrarily, but this is something only known by Transport for London and isn't widely publicised, for security reasons.

Obviously, sometimes station buildings go above ground so the top number changes to a letter, beginning with 'A', then 'B', etcetera.

Now, if there is ever an emergency, there is an emergency box containing detailed plans of each station complex, and these plans have the SIDs on them, making finding a specific location easier to find and reach.

Although these SIDs are designed for use by Transport for London staff and the emergency services, they are also useful for passengers to use, should they be in difficulty or need assistance.

So the next time you are travelling on the tube keep your eyes open for these little blue plates, as they are more important than you think.