Wang Fuchun has been a "thief" on trains since the late 1970s.

Wang is no pickpocket -- long employed by the state to photograph life on the train network, he characterizes himself as stealing the images because he believes acting without his subjects' permission is the only way to capture reality.

With his camera generally covered by a coat, Wang has prowled through packed carriages snapping what he sees as his "prey" on a journey that has allowed him to trace China's modernization and great cultural change.

Now, however, he is ditching the covert tactics and has launched a project to identify the people in his old photographs. Wang, 73, wants to find out what has become of the unknowing stars of his albums.

"There's a mobile community on the train, and it has included every aspect of society; the good, the evil, the beautiful and the ugly," he says. While Wang yearns to get to know the subjects, he believes the images are powerful in themselves as historical snapshots.

In a photo taken in 1992, a naked boy stands on a passenger seat peeing into a tin can.

"Little boys often used to strip off before trains had air conditioners installed," Wang explains. His photos also show lots of topless men in the stuffy compartments.

His pictures also give a peep into Chinese fashion trends: In the 1970s, people wore uniforms. In the mid 1980s, bell-bottom pants became in vogue, and the 1990s witnessed a prevalence of T-shirts. FULL STEAM AHEAD -- FOR BETTER OR WORSE

Among all the changes, what impresses Wang the most is the improvement of the travel experience.

Slow, shabby trains used to be jam-packed. "It took half an hour for people to jostle through the crowds to the bathroom a dozen steps away," Wang recalls. His albums have scenes of people clambering into packed carriages through windows.

These scenes were most common during the Chinese New Year holiday rush, often described as the biggest human migration on the planet.

"The painful experience disappeared along with the development of the high-speed railway," Wang says. Passengers can now enjoy far more comfortable journeys in modern trains on the world's longest high-speed rail network.

But for Wang, the improvement has also brought unwanted side effects. He misses people sitting besides each chatting to kill time. "You could make friends on a journey," he says, "but now people just stare at phones or laptops. They seldom talk."

He can't help to ask: "Is technology bringing people closer together or pushing us apart?" NO TERMINUS

Wang grew up dreaming of becoming a train driver. After school, he got a job in a train station in his hometown -- Harbin, in northeast China's Heilongjiang Province. He took his first picture in 1977 when asked to photograph model railway workers.

Wang's approach to photography has got him into trouble on numerous occasions. He has been reported to police by passengers suspicious of his snooping around, and he was beaten up in July by a man who objected to his baby being photographed.

Undeterred, Wang "will keep on photographing people on the train for the rest of my life."

Wang moved to Beijing in 2002, the year he officially retired. He experienced deja-vu on the subway and could not resist continuing with his vocation.

"Taking photos over such a long span of time has been fascinating," he says. "That naked boy will have grown up to be a man. I wonder where he is now?"