I wouldn't count on bringing a city down that way. The Brits slept in the underground during the blitz. I believe the subways under New York are cut through granite. The subways under Moscow are the deepest in the world due to the protection the ground offers.
All bombing a subway does is kill people.
The subway tunnels are not cut through granite. City streets were build above them, rather than the subways being built below them.
If you damage the tunnel seriously enough, the line will be taken out for a while. 9/11 resulted in closures for months, of tunnels in TriBeCa / lower Broadway. It may take a while, even though the explosion Thursday was - relatively - a dud, to get the Picadilly Line open again in that area. The tunnel is currently shored up and there is still a concern about collapse.