A London Underground code alert is a warning issued by a London Underground Line Controller (who oversees the running of a line in conjunction with the signal operator) over the radio system giving both train and station staff information about a potential problem. The codes are rarely used as they are specifically reserved for emergency situations.
Codes
A code amber alert requires all trains to go no further than the next station as far as possible while obeying all signals. The order stops the trains but permits the rapid evacuation of carriages and stations and the rerouting of passengers. A code amber alert was issued in response to both the London bombings on 7 July and 21 July in 2005.
A code red alert is an immediate stop order issued to all trains irrespective of their locations. This could result in getting passengers out from trains stopped away from platforms or part-way down tunnels. In these cases, special procedures would be required to evacuate the carriages when considering the presence of electrified rails in the tunnels.
An Emergency Code Evacuation is issued when a specific area has to be evacuated. This code is used to evacuate a section of the line when life and safety is in direct peril. This code allows abnormal movement of trains (For example: trains moving in the wrong direction to reach a station.).