Clearly the underground could be covered by the mobile networks.

BUT what about public safety? The Madrid bombs were denotated by mobile signals. This can't currently be done over most of the tube as there is no coverage. This BBC report says coverage in NYC tunnels has been cut for this very reason: http://news.bbc.co.uk/1/hi/world/americas/4674731.stm

Early reports on 7/7 said that the mobile phone networks in central London were unavailable immediately after the bombing. Was this true? Were they turned off, switched to only allow high access control classes or did they go down under traffic load?