Long working hours meant he seldom played with his daughter. He decided to use the augmented reality technology that he and his colleagues used as a marketing tool for companies such as L'Oreal and Mercedes-Benz to "build" a city gate.

His idea resonated with his team of more than 20 developers. Like Li, they mostly hail from other parts of the country, and with an average age of 28, they had little time to study the history of their adopted home city.

In November, Li and his colleagues invited artists to paint pictures of the nine lost gates, based on written materials they discovered online, then posted them in stations. The group invited subway commuters to scan the pictures to get a visual and aural description of the area they were passing through. The visuals were accompanied by a short commentary on the vanished buildings.

When the project was launched in December, passengers loitered in crowded stations and tried the technology. Some enjoyed taking selfies with the virtual models.

In late January and February, a painting was posted at Chaoyangmen subway station on Line Two.

The project can now be seen at the Beijing Planning Exhibition Hall at an exhibition called Technologies Awaken City Memories.

"It (augmented reality) helps people to learn about the city's history and the changes made," Hu Daxin, the hall's deputy curator, said. "The more you learn, the stronger your sense of belonging."

As Beijing has expanded and its population has risen, the subway's tentacles have stretched further and become more complex. Currently, there are 19 lines, covering 574 kilometers.

The demolition of the city gates and walls to build subways and roads has been a controversial topic for decades.