CHONGQING, Jan. 8 (Xinhua) -- An operational failure at a metro loop line in southwest China's Chongqing Municipality left four injured, according to the city's rail transit company.

The failure was caused by a breakdown along two stops of the line after an interruption to the subway's protective doors.

The four injured -- one passenger and three staff members -- have been sent to hospital. The remaining 30 passengers on board were evacuated.

The Chongqing Rail Transit Loop Line was put into use on Dec. 28, 2018, and its construction began in 2013.