Last weekend's train crash in China was caused by a "serious design flaw" in the signals, officials say.
Thirty-nine people died when one train ran into the back of another, which had stopped on a viaduct near Wenzhou after lightning had cut its power supply.
The system "failed to turn the green light into red", An Lusheng, head of the Shanghai Railway Bureau, said, according to Xinhua news agency.
Chinese Premier Wen Jiabao has called for a swift and transparent inquiry.
He has also promised to take steps to improve safety on the high-speed rail network - one of the government's flagship projects.
The public has expressed outrage at the accident, and many people believe the government has mishandled the aftermath of the crash.
The premier is due to visit the crash site shortly.
.jpg)
