China says two subway trains have crashed at a station in Shanghai, injuring more than 270 passengers.

The official Xinhua news agency said one train slammed into the back of another after a mid-afternoon signal equipment failure at a station.

The report said most of the injured sustained bone fractures and head trauma. Twenty people were reported in critical condition hours after the crash.

Xinhua published photos of injured passengers being removed from a train on stretchers. Pictures of bleeding and apparently unconscious passengers were posted on Internet microblogging sites.

The news agency said the crash was the third system failure on the subway line in the past two months. In one of the two earlier incidents, Xinhua said signal failures sent a train heading in the wrong direction on July 28. Five days later, another train stalled on the same line. No casualties or injuries were reported in either incident.

Tuesday's accident had similarities to a train wreck in July that killed 40 people and shattered public confidence in the nation's high-speed rail system.

That accident, in the city of Wenzhou near Shanghai, also involved a rear-end crash and was blamed on equipment failures.

China has poured billions of dollars into its scandal-plagued bullet train network, prompting critics to complain that Beijing has sacrificed safety in its quest for rapid economic growth.

The July crash prompted an unusual outpouring of public anger on social media and even in the normally supportive state-controlled press.