A San Francisco Municipal Railway train carrying passengers rolled out of a station today leaving behind one important person: the driver.

The train entered a tunnel between the Castro and Forest Hill stations when a passenger noticed the driver wasn't inside the train and activated the emergency brake, according to the Associated Press.

John Haley, director of transit for the agency, told the AP that passengers were never in danger. He said the train's automatic control system was remotely activated when the doors closed, which allowed it to arrive safely at the next station.

Operators are required to put on the emergency brake before leaving the train, Haley said. An investigation is underway to determine whether the driver forgot or if the train has mechanical problems.