Last Wednesday, an alarm that alerts authorities about possible obstructions on San Francisco’s world-famous cable car line didn’t sound before a cable car hit a bolt and came to an abrupt stop, injuring seven people. The alarm didn’t sound because the loose bolt came from a street surface plate that was lodged into the tracks, San Francisco Municipal Transportation Agency spokesman Paul Rose told The Associated Press Thursday. The bolt was above the underground cables that have sensors around them, he said. Wednesday’s mishap occurred when a Hyde Street line cable car, heading north at about 9 mph on Powell Street just past Washington Street at 10:15 a.m., hit the 1½-inch bolt. The bolt had fallen into the channel containing the cable that pulls the car. After it hit the bolt, the cable car came to a sudden halt as it was going downhill. About 40 people were on board the car at the time of the accident. The agency plans to conduct weekly checks on each of the bolts and the panels along the renowned five-mile cable car route “to make sure they are secure as possible,” Rose said. Transit director John Haley said Wednesday several items including those as light as pieces of paper in the past have set off the system’s alarms, so it isn’t clear why the metal object did not. Among the seven injured were a conductor who underwent surgery Wednesday for facial and tongue injuries and a gripman who suffered internal injuries and cracked ribs, transit officials said. Five were taken to a hospital, including an elderly man in his 80s with a potential life-threatening head injury, authorities said. Also, San Francisco police reported a female passenger suffered a leg injury when she was thrown from the cable car.