Actually the underground is not in rock. The tunnels were dug through a deposit called the London Clay. The clay is from the deposits of the Thames River. It would take a hell of an explosion to go through the clay and into the Thames. The clay is plastic in nature and therefore self sealing. For this to work they would literally have to blow a crater up to the river bottom. The nauture of explosions would make most of the explosive force follow the path of least resistance which would be down the tunnels.
Then there are the odds to consider:
1) The probability of their overall mission success,
2) The chance that out of 100's of thousands of commuters that any individual will be affected,
3) The probability that the explosion will occur at the right location,
4) The probability that intelligence will not get them before they get us.
I would say the odds are in the favor of any given commuter.