The Beijing Subway is the world's busiest subway system, with an average of more than 10.5 million passenger trips made every day.

Line 10 has the most transfer stations among Beijing's 22 lines. It's also the place where Ding Chen has worked as track inspector for the last 14 years. "It ensures that the public is safe," said Ding.

When the last train on line 10 stops operating, Ding Chen and other subway workers begin their work. They step into the dark tunnel to make sure this giant transportation system keeps on going.