Serious flaws in a signalling system caused a fatal collision on China's high-speed rail network, officials say. Thirty-nine people died when a train ran into the back of another which had stalled on a viaduct near Wenzhou after lightning cut its power supply. The system "failed to turn the green light into red", said An Lusheng, head of the Shanghai Railway Bureau. Chinese Premier Wen Jiabao, who has been visiting the crash site, vowed to "severely punish" those responsible. Wen Jiabao held the news conference under the viaduct where six of these carriages came off during Saturday's collision. He stressed that safety would be the absolute priority as China built its huge high-speed network. It is already the second-largest in the world and is set to be expanded.