It just popped into my head.
Let's say you're a terrorist organization. Not one of those eastern ones - a western one. In North America or Europe. And lets presume that the government has a lot more monitoring capability than I give it credit for.
The "law" can track what you're communicating, what you're doing, to some extent. They probably hear your phone calls and read your emails, since both are suspicious or have been suspicious. They follow you, mister suspicious-pants, around. They don't want you blowing anything up.
But your goal is to blow shit up. And you're good at it. You want to get together with your friends and blow all the shit up. You want to, I dunno, smuggle in a nuclear bomb or a sizeable amount of chemical weapons to be distributed by toy aeroplanes.
You can't really chat about these plans with your buddies, because anytime you say anything suspicious or gather in one place, the gov comes down on you, WHAMMO. They're watching for key words like "chemical weapons", "bring over the border", and "kill ten thousand people".
So, what do you do?
You blow up a few subway cars.
Honestly, this isn't all that hard. It doesn't even require a whole hell of a lot of coordination. You meet with your terrorist buddies and say, "plant a bomb on something that moves. Set it to go off Thursday, at 8 AM."
A few days later, kablooie.
A wise city government would NOT stop public transport, save for that required to route around damaged zones and crime scenes. The damage is already done. But the city government is not wise. They'll shut everything down.
The whole country will buzz with chatter about terrorism. The government will be in a panic, rushing around, desperately trying for spin. Desperately trying to catch your four friends who you got to plant explosives. Not that they WILL, but they're TRYING.
During this massive glut of concern, you are clear to chat with your buds and plan a REAL operation instead of this nickle-and-dime shit. The city is frozen up and is receiving roughly fifty calls a second containing the keywords the government normally monitors. You pop on the horn and arrange to smuggle in one of the aforementioned nasties.
This has to happen quick: you can't wait, because after the attack, security will gel and harden, and you'll have a bitch of a time. Within two days of the attack, a SECOND attack happens, this one involving military-grade unconventional weapons. Or even just involving fifty bombs instead of four.
Because you've turned their strength - their ability to scan your communications - into a weakness by simply bringing ten million people down on it. You have turned ten million unwitting innocents into soldiers for your cause.
I'm not saying that is what happened. I would be shocked if it were. But it is a nasty idea that would probably work.
I'm also not saying that crippling the London transport network is 'nickle-and-dime shit'. But it isn't anywhere near as bad as a concerted terrorist attack would be. London will suffer economically and politically, but there aren't very many people DEAD in comparison to what could have been.
As a side note (if something like this can be called a side note):
Meme propagation being what it is, I am very worried about the concept of "bombing subways works" spreading.
London needs to come down HARD on this in three ways.
First: they need to annihilate someone. They need to find someone to blame and SLAM them down. It's better if they find someone who WASN'T actually responsible.
Second: they need to do it with an icy calm and a minimum of news coverage. Just enough to spread the idea that, yes, the government has slammed them and, no, we're not going to talk about it. IE: Don't cover the attacker's agenda, don't talk about how they did it, just put their faces on the screen and execute them.
Third: they need to not only repair the damage to their transport systems, but IMPROVE their transport systems.
This will give the terrorists only negative feedback. If London manages to suppress the media bloodbath that usually follows this sort of thing, the meme won't spread nearly as far. You do NOT want bus-bombing becoming the bomb-du-jour, because every bomb which damages public transport is a direct hit to your economy.