The official Xinhua news agency said the track work had been scheduled to start at 10pm and that trains had been told to slow their speed to 45kph from an hour earlier, but that the workers had arrived early and entered the work area at 8:40pm.
A spokesman for the engineering company involved blamed bad weather at the scene.
"It's snowing heavily in the area and traffic and telecommunications are very bad," the spokesman told AFP.
China's rail network is gearing up for the busiest travel period of the year as hundreds of millions of people head home to be with their families for the Chinese lunar new near holiday.
The accident comes just days after the government announced a major crackdown on workplace safety.
On Tuesday Wang Mei, China's works minister, said official figures showed more than 100,000 Chinese died in workplace accidents in 2007.
He said managers who cut corners over safety would face prosecution.