In a statement, a spokesperson for Stockholm Public Transport said the driver of the out of service bus had taken a different route than he had been instructed to take.
He chose to go via the Klara tunnel route, which is open only for vehicles lower than 3 metres, and the gas canisters that were placed on the roof of the bus banged straight into a metal barrier hanging above the tunnels entrance as a precaution.
"It is a good thing that it exploded outside the tunnel instead of inside it, said Erik Egart, fire protection engineer with the Civil contingencies agency MSB.
Experts are now analysing what can be done to prevent a similar incident in the future.
The driver escaped with only minor injuries, according to his employer Keolis.