[lepton]

I'm also curious about the bomb on the bus. Since the others were all on trains, I'm wondering if that was supposed to be on a train as well, but for some reason the perp was delayed, or redirected, or some such.

From earlier explanations, the buses were packed because the tube had been shut down. It went off nearly an hour after the first blast, and half an hour after the previous blast.

[Myrddin]

From earlier explanations, the buses were packed because the tube had been shut down. It went off nearly an hour after the first blast, and half an hour after the previous blast.

That is similar to the strategy of performing a small act of terrorism to attract first responders and a curious crowd. When the crowd has gathered, a much larger bomb is detonated to nail the large assembly of potential victims. Stopping the "tube" caused a predictable alternative flow to fill the buses.