Beijing’s subway network is the busiest in the world, with commuters taking about 10 million rides a day throughout nearly 200 subway stations.

These commuters, who already deal with massive traffic jams, overcrowded public transportation, and air so filthy that biking or even walking to work is often not an option, are experiencing a new obstacle: airport-style security at major subway stations.

On Saturday, Beijing tightened security checks at subway stations following an attack in China’s troubled Xinjiang region which killed 31 people.

Here are photos from this morning’s massive lines during rush hour at Beijing’s Tiantongyuan North Station.

Jason Lee/Reuters

Jason Lee/Reuters

Jason Lee/Reuters

Jason Lee/Reuters

Jason Lee/Reuters