Well, at the Saturday press conference two days after the bombing Tim
O'Toole stated that the Piccadilly line tunnel was intact,

Just from what I've seen in the media and by the consequences of the
bombs it seems to me that they weren't all that powerful (otherwise
the carriages would have completely annihalated and no one inside
would have survived) so I suspect damage to the tunnels (which are
built a lot stronger than the trains) is probably limited to damaged
cabling and track.

B2003