What do the codes on the maps mean?

Each of the boxes on the map which can hold a code is called a berth. Codes can be up to 4 characters long, but longer messages are often displayed by using berths next to each other. It's important to bear in mind that our maps are often the other way up from the maps in use in signalboxes. This will lead to a 2-part message appearing backwards!

We also often combine paired berths into one to keep the map nice and compact, and show the codes on top of each other if there is more than one, so two-part messages can also read vertically, either from top-to-bottom or bottom to top.


Most of the codes on the maps refer to a train. Any code in the format 9A99 or A999 is probably a train.

The first format (A999) is used for trains where Network Rail do not have permission from the operator to share the identity of the train. Generally these are freight trains or trams. Due to glitches in the system other trains may occasionally appear in this format. Sometimes these trains can be linked to a schedule in the database, but this depends on whether the schedule information has been obscured in the same way.

The second format (9A99) is a code for a train called a headcode. Very roughly the leading digit indicates what type of train it is, with 1, 2 or 9 generally being passenger services. 5 (and sometimes 3) indicate a passenger train being driven empty to or from the depot. 0 indicates a locomotive running without a train. All other codes indicate a freight or maintenance train. You can learn more about headcodes from Wikipedia.

If a headcode or obscured code can be linked to a train in the schedules database, it will be displayed on the map in a red, yellow or green colour indicating if they are on-time or how late the train is. If you click on it you will be shown a summary of the schedule. Codes which cannot be linked can still be clicked, and a list of possible candidates will be shown.

Line Blocks and Possessions

Individual signalboxes have different procedures for marking blockages. Sometimes it is done manually with paper and physical clips, so the data feed is not updated at all, and sometimes the data provided in the feed is just generally helpful information which isn't critical to the signaller's work.

**** indicates that the track circuit for a section indicates that the line is occupied, but it is unknown what is in the section

-LB- simply indicates that the line is blocked, and the blockage is known about by the signaller. These codes are input manually by signallers, and sometimes the thing blocking the line may be described slightly more poetically, with TREE, ROOF and TRMP OLNE all having appeared in the past.

-T3- is a type of planned possession, where a section of line is closed in order to allow maintenance work to be carried out. The markers for this will generally be placed at each end of the blocked section, often with a numeric code next to it indicating where details of the blockage can be found in the operating notices. A 24hr time may also be placed indicating the time at which the possession is scheduled to end.

Shunt Moves and Stabled Trains

The way that shunt moves and stabled stock are displayed within the system varies significantly from area to area, so this is simply a guide to what might be happening.

A train which is destined for another platform will often have a platform number in the form P18, PL18 or PLT2. If it is bound for sidings it may have something like SDGS, or DEPO for the depot. Any train with a code like this which is actually in the correct place has probably already been shunted, and is now being stabled in the designated location.

Trains which are stabled in platforms or sidings will often be designated by their length 2CAR, their unit type UNIT, EMU, C350 or 2X C350, or their scheduled departure time in the morning. Locomotives stabled in sidings are often described as LOCO, LECY or TBRD (the latter being a rescue locomotive).

General Notes and Messages

Sidings or links to other boxes are often marked OPEN, CLSD or FULL. If a siding or line has been removed from the network altogether it might be marked as GONE or in at least one case GONE 4EVA. Most prosaically, when the through platforms at London Bridge were being developed the missing lines carried the message ONE BIG HOLE.

Where lines are closed for christmas it isn't uncommon to see christmas greetings in the berths, which can be fun to look out for when boxing day starts to drag. As boxes close permanently memorials may be placed in the now-out-of-use berths, although generally you won't see these since they are usually replaced very quickly on the maps with the new signalling system!