Investigators immediately focused on whether excessive speed or the actions of the driver caused the crash in an urban area near Amagasaki, about 250 miles west of Tokyo.
The investigation is really focusing on the driver, reports CBS News correspondent Barry Petersen. This was a young, inexperienced driver -- he'd been on the job less than a year. Train drivers in Japan are under intense pressure to get to the stations exactly on time. To be late is to be in trouble with your boss, and this train was running late.
The 23-year-old driver had overshot the stop line at the last station before the accident.
The tracks do have safety equipment that will slow the trains down even if the driver does not, but in this case the equipment was not working, Petersen reports. This is one of the oldest tracks in Japan, and that surely will also be a focus of the upcoming investigation.
Rescuers were trying to free four people found alive in the wreckage more than nine hours after the crash, said Yoshiki Nishiyama of the Amagasaki fire department. They were trapped in one of the two worst-damaged cars, but their conditions were unknown.
The seven-car commuter train was carrying 580 passengers when it derailed at 9:18 a.m., wrecking an automobile in its path before slamming into a nine-story apartment complex just yards away. Two of the five derailed cars were flattened against the wall of the building, and hundreds of rescue workers and police swarmed the wreckage and tended to the injured.
"There was a violent shaking, and the next moment I was thrown to the floor ... and I landed on top of a pile of other people," passenger Tatsuya Akashi told NHK. "I didn't know what happened, and there were many people bleeding."
The Amagasaki Fire Department said at least 50 people were killed. It was not clear how many of the dead were passengers or if bystanders or apartment residents were among the victims. Train operator West Japan Railway Co. said at least 343 people had been taken to hospitals.
The accident was the worst rail disaster in nearly 42 years in Japan, which is home to one of the world's most complex and heavily traveled rail networks. A three-train crash in November 1963 killed 161 people in Tsurumi, outside Tokyo.