A subway operator in Shenzhen in Guangdong province is investigating whether passengers' portable Wi-Fi devices were the cause of a brief suspension of metro services on Thursday.

"Unknown signals", as the Shenzhen Metro Group put it, interrupted several trains on the city's Shekou line from 8:15 am to 9:30 am on Thursday, forcing many passengers to transfer to other lines. Others found themselves stranded.

The incident has attracted nationwide attention as more cities in China have started operating subway services. There has been much discussion as to whether portable Wi-Fi devices should be banned on metro lines to ensure their safe operation.

Because of interfering signals, the trains on the Shekou line stalled a number of times under the automatic protection mode or failed to operate at full speed.

Concerns rise over the Communication Based Train Control System - a railway signaling system that uses the telecommunications between the train and track equipment for traffic management and infrastructure control - which may be affected by Wi-Fi signals.

Chen Qi, deputy manager of the Shenzhen Metro, said on Monday that portable Wi-Fi devices may be the cause of the disruption, but there has been no final conclusion.

According to the operator, the Shekou line and Huanzhong line in Shenzhen use the CBTC system.

The incident has been the cause of much concern around the country, as first-tier cities such as Beijing and Shanghai are expanding their metro services, and second- and third-tier cities are building their own subway systems at a brisk pace.

Metro companies in a number of cities approached by China Daily on Monday said they never had services interrupted by unknown or Wi-Fi signals.

Shanghai Shentong Metro Group, the main operator of Shanghai's subway lines, said no signal interference has ever occurred on metros in the city despite metro lines 6 to 11 using the same CBTC system as their wireless communication system.

"But the interference is possible in theory," said Yang Di, a manager at Shentong.

Jia Peng, spokesman for Beijing Subway Operation Co, said the signal system of subway networks operated by the company never experienced problems like the case in Shenzhen and he firmly believes in the safety of Beijing's subway signal system.

"The CBTC system we use is the most widely used signal system in the world and also the most tested," he said.

Despite the confidence of some metro operators, mobile communication experts said it remains a concern that portable Wi-Fi signals can be a potential hazard to subways using CBTC systems.

Contact the writers at [email protected] and [email protected]