Lines are marked in four colors — black, red, yellow and green — depending on the number of people on board.[Photo provided to chinadaily.com.cn]

Beijing Subway has started to release real-time information on how crowded its 19 lines are to help passengers beat the crush.

The service is already available on its website and will be added to its smartphone app soon.

Lines are marked in four colors — black, red, yellow and green — depending on the number of people on board. Black indicates very crowded, while green indicates few passengers.

Stations closed temporarily due to high passenger numbers are marked with a red and white fence icon.

The information will be updated every five minutes, according to a Beijing News report.

"Beijing has 19 subway lines and over 300 stations, and the information is going to help passengers avoid the busiest lines by using a different subway route or by using ground transportation," a Beijing Subway employee told Beijing News.

Li Lei contributed to this story.