I'm not sure. The subways may be too far underground. I don't think they pick up the signal. That's probably why the London terrorists used timers. I think their subways are further below ground than NYC's.
In a Second Communiqué from the Abu Hafs al-Masri Brigades the Group Claims: London Attacks Were the Start of a “Deadly War”, and Cites Denmark at the Top of a List of European Countries with Soldiers in Muslim Lands By SITE Institute