The Shenzhen Metro is facing mounting calls to resolve questions about its wireless control system, after a train came to a halt in between stations for the third time in a week.
In the latest incident, a train on the Huanzhong Line stalled twice in a tunnel between the Changlong and Buji stations during the morning rush hour on Wednesday, the Southern Metropolis Daily reported.
The train stopped at 8.40am for about two minutes before starting and stopping again.
The breakdown came after the Shenzhen Metro blamed interference by signals of passengers' Wi-fi enabled mobile phones with similar incidents on the Shekou Line on Monday and the previous Thursday. Mainland media has blamed the problem on the transport company's cost-saving move to operate its trains using the publicly- available 2.4-gigahertz wireless band - which is also used by consumer electronics.
It was unclear if the same issue contributed to the problem on the Huanzhong Line.
Neither the Shenzhen Metro nor the city's telecommunications authorities were available for comment yesterday.
Several passengers took to Sina Weibo to complain about being trapped on the train and to criticise the company's silence. Some said they struggled to stay on their feet as the train came to a sudden stop.
"I was trapped in the train twice," one Weibo user named Ranranchen09 wrote. "Who will tell me what happened?"
Some passengers said the problems predate the reported incidents, citing similar cases on September 5, October 12 and October 31.
"Tonight at 10.10pm … the train suddenly stopped at the tunnel between Hubei station and Huangbeiling station … passengers shocked … I felt scared when the train restarted without any warning," Ms Lu wrote on September 5.
Several passengers have posted on the Shenzhen Metro's official Weibo account, demanding an explanation and an apology.
Telecommunications authorities reportedly rebuffed a request by the transport company on Wednesday to cut 3G services inside the subway. They said the metro should have bought a licence for a wireless band less vulnerable to interference.