The International Workers' Day on May 1 has never brought subway driver Kuang Wentian, 34, much respite from the pitch dark of the tunnel. On the contrary, increased traffic on the national holiday brings him even more stress.

On Thursday, this year's Workers Day, Guangzhou, capital of Guangdong Province, expects a record traffic of 7.7 million passengers on its metro.

The stress Kuang endures is common among subway drivers in China, which has seen fast expansion of underground networks in major cities. As of 2020, 50 cities are expected to have subway networks, compared to 19 at the end of 2013.

Being aware of the problems drivers face, some metro operators give psychological help to subway drivers prevent long-term health problems.

NEGATIVE EMOTIONS

Over the past 16 years, Kuang has covered more than 500,000 kilometers, equivalent to more then 12 times around the world, and carried tens of millions of passengers without accident. Despite this perfect safety record, Kuang can never relax during tedious daily routine. He is forever pulling hand levers, signaling, monitoring, opening and closing doors, and watching passengers in the carriages and on the platform.

"The biggest headache is equipment malfunction. We're required to comfort the passengers, find the problem and fix it, all within three minutes," he said.

As a result of soaring passenger numbers, the interval between trains has been reduced to less than three minutes, which is a big safety issue for drivers, said Huang Ping, a senior manager with Guangzhou metro.

For Kuang, the loneliness he suffers underground every day is worse than the stress. "You sit through eight hours in the tunnel all alone, and must keep alert all the time," he said, adding that he spent New Year's Eve in the noisy cab many times.

Occupational psychologist, Song Guoping, believes subway drivers feel isolated because they have few chances to interact with workmates and many are dissatisfied with their pay and career prospects.

"These feelings are likely to take a toll on the drivers' personal lives," Song said. "For example, some take their silence in the cab back home, while some are just the opposite, talking too much after work."

A regulation that took effect on April 1 this year means that subway drivers must take psychological tests on a regular basis.