I am curious about the details of this plan. I am sure the tunnels are far beneath the river, and it would take a lot of explosives to crack the rock above. I would think the police/subway employees might get suspicious if someone tried to bring a thounsand pounds (or kilos, in this case)of packages on a single car.
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.