Service on a Shenzhen subway line was halted this week, reportedly because the system had been compromised by a Wi-fi signal operating on the same frequency.
The signal may have caused interference that interrupted services and possibly made the subway system vulnerable to cyberattacks, one expert said.
The brief suspension on the Shekou Line on Monday morning followed a similar incident on Thursday, in which the line's service was interfered with by signals that triggered an automated safety measure on the train and prevented them from operating at high speed, according to The Southern Metropolis Daily.
The newspaper cited sources from Shenzhen Metro in reporting that the line's operator suspected that the interference came from passengers' Wi-fi-enabled mobile devices, especially those used to connect to 3G phone networks for sharing internet connections.
The report also said Shenzhen Metro had asked the city's telecommunication authorities and several mobile phone network operators to "temporarily" cut 3G services inside the subway, but received a cold shoulder.
Publicly available Wi-fi operates on the 2.4-gigahertz radio frequency. Mainland media reported that subway systems in major cities, including Shenzhen and Guangzhou, used the 2.4GHz frequency band to transmit signals for the trains to run.
"It's cheap for the MTR subway companies to use the public frequency. It's free with no need to apply for a licence," said Gao Zehua , of Beijing's University of Posts and Telecommunications.
But Gao said using public frequencies for public transport systems was unsafe. "MTR companies should change to a different frequency. It may be much more expensive than using public frequencies, but safer."