SHANGHAI - A subway train rear-ended another Tuesday afternoon in Shanghai, leaving more than 260 passengers, including four foreigners, injured.

Most of the injuries are bruises and bone fractures, but there are also external head traumas, doctors said. An estimated 20 of the injured are in critical condition but the injuries are not life threatening, they said.

The crash occurred at about 2:51 p.m. following a signal system failure at a station on the Line 10 subway, Shanghai Shentong Metro Group Co. said in a statement, adding that about 500 passengers were later evacuated from the trains.

The subway train stopped for about 15 minutes and then went on before stopping again for another ten minutes before crashing into the other, a young passenger on the train's first carriage said.

The signal system failure at about 2:10 p.m. meant the trains had to be directed via phone by subway staff rather than by electric signals and thus were running at slower speeds, the subway operator's statement said.

The signal system is a product of Casco Signal Ltd., a joint venture of China Railway Signal and Communication Corp. and Alstom, which reportedly supplies signal systems to a number of subways in Chinese cities including Beijing, Shanghai, Tianjin and Shenzhen.

Casco was blamed for a subway train crash in Shanghai in 2009.

Casco also provided the centralized traffic control system for a railway in east China's Zhejiang Province where two bullet trains crashed on July 23, killing 40 people and injuring 177.

Jiang Jianhua, Casco's chief engineer, could not be reached for comment. A staff member at Casco's headquarters declined to confirm with Xinhua if the company had launched an investigation into the accident.

The company's website www.casco.com.cn could not be opened on late Tuesday afternoon.

Local authorities have launched a further investigation into the crash.

Photos posted on weibo.com, China's popular Twitter-like microblogging service, showed several passengers bleeding, with firefighters entering the train to rescue the injured.

"I was stunned, not knowing what happened. We tried to open doors and windows but couldn't. I felt smoke in the car," a passenger said shortly after being rescued from the train.

"The train braked suddenly...some people fell and some cried, then the automatic alarm sounded," said another passenger surnamed Bian.

Four foreigners, including two from Japan, one from Canada and one from the Philippines, suffered minor injuries during the accident and have received treatment at hospital, according to the Shanghai Municipal Bureau of Health.

As of 7 p.m., the scene was cleared, and the Line 10 started to resume operation, said the the subway operator. Earlier subway services at nine stations on the Line 10 were halted.

The subway operator offered an apology via its verified weibo.com account at 8:20 p.m.

"Today is a dark day in the history of Shanghai Metro. We feel deeply sorry for the injuries and losses of the passengers no matter what the investigation results will be," said a brief statement from "shmetro."

The crash was the result of a third system failure on the Line 10 over the last two months.

Signal failures guided a train in the opposite direction on July 28. Five days later, another Line 10 train stalled in tunnel after a controlling device breakdown. No casualties or injuries were reported in the first two accidents.