This from ABC News:
Sources familiar with the investigation tell ABC News an additional 12 bombs and four improvised detonators were found in the trunk of a car believed to be rented by suicide bomber Shehzad Tanweer. Police believe the bombers drove the car to Luton, where they boarded trains to London.
“I believe that the explosives left in that car were left there for a second strike,” said Bob Ayers, a London-based terrorism consultant with expertise in demolition. “But the metropolitan police responded so quickly, they were able to get to the car and take control of the car before the second team could get the explosives and leave.”