The boot of the Volkswagen was packed with 100kg of stolen industrial dynamite, according to an El Pais newspaper report.
Three of the four train-bombing suspects were thought to have been in the car at the time, but its Arab driver was fined for a minor traffic offence and allowed to drive on.
The Telegraph, quoting the Spanish newspaper report, said two patrolmen who stopped the car north of Madrid failed to realise that the driver was not the registered owner. The owner of the car had not yet reported it missing.
It has now been found, along with traces of the dynamite used in the March 11 train bombings that killed 202 people.