As it scrambles to prevent more doors from closing on riders’ hands, Muni is dealing with a new threat that train cars can open unexpectedly as well.
NBC Bay Area’s Investigative unit first reported about an incident Tuesday in which a door on one of Muni’s old fleet of Breda cars closed on a woman’s hand as she was trying to get out of an N-Judah car. That follows three incidents in which riders have gotten caught in doors of the new trains since October. But there is a new treat, as revealed in a briefing paper to Muni’s governing board on Thursday and obtained by NBC Bay Area.
In the document, Muni officials described how a train car door opened suddenly inside a crowded Breda train car on May 17.
That door is supposed to remain locked anytime the train car moves, to protect passengers from falling out. But it suddenly opened after a passenger in the crowded car pushed the door opening bar – a bar that should not have been functional with the train in motion.
But the door opened and that triggered the train’s automatic braking system. The door closed when the bar was released.
Muni engineers did an investigation and quickly spotted a loose control wire in the system. That loose wire, Muni authorities concluded, “made the door controller device think that the vehicle was stopped, so the door circuits continued to behave as if the vehicle were stopped, even as it was moving.”
Muni engineers found they could recreate the problem. With a loose wire, they found, just routine service vibrations could trick the door controller into concluding a moving train was not moving and allow the door to open.
“We have never seen this failure scenario happen before,” Muni’s engineers said in a report to the board, adding that the wire connection, which had not been checked in the past, will now be inspected during routine maintenance checks every 20,000 miles of service.
In the meantime, Muni will inspect the wire connections on its entire aging fleet of more than 148 other Breda cars by June 5. But the issue has not been found in the 14 cars inspected so far.
Meanwhile, Muni is also inspecting 60 of its cars for more cracks like the those they found this week in the pressurized air-filled tube of the door system that closed on the woman’s wrist on Tuesday. The inspections are limited to car door systems that Muni has not previously overhauled.