Well, perhaps after the July bombings, he means that the more packed a train is, the more difficult for the suicide bomber to get their hands to the detonator.
Anyway, Londoners complain about tiny little things like busy trains.
The yamanote line has to be at least twice as busy as any London train.
Perhaps if the trains in London actually ran on time, there wouldnt be such a backlog of passengers piling on, and making it so damn busy.