A 16-centimeter crack recently found in the undercarriage of a running shinkansen bullet train could have been serious enough to cause a derailment, train operator West Japan Railway Co. said Tuesday.

"It was an extremely serious incident that has affected trust in the safety of shinkansen," JR West Vice President Norihiko Yoshie said, referring to what the transport ministry's accident investigation board has determined to be the first "serious incident" affecting the Japanese high-speed train system.

"It could have caused a derailment," he told a press conference.

A railway safety expert echoed the view, saying the steel frame holding the set of wheels, where the crack was found, was "close to breaking apart." "If the train had continued running, there was a high possibility of a major accident," he said.

The crack was found in the undercarriage of the Nozomi No. 34 bound for Tokyo from Hakata, Fukuoka Prefecture, after the crew had noticed a burning smell and heard an abnormal motor sound on Dec. 11.

The bullet train was halted at Nagoya Station after having run more than three hours with the abnormal smell and noise.

"We must take a hard look at ourselves in that we did not stop the shinkansen immediately despite the abnormalities," Yoshie said, expressing disappointment especially as the company has vowed to make safety its top priority following a 2005 derailment that killed 106 passengers and the driver in Amagasaki, Hyogo Prefecture.

The crack at the bottom of the steel frame was 16 cm long and there were also 14-cm cracks on both sides of the frame, the company said.

The train was moved from Nagoya Station to a carriage base in Hakata for a probe after occupying one of the tracks for a week. The transport ministry's Japan Transport Safety Board and Kawasaki Heavy Industries Ltd., which made the train's undercarriage, are also investigating what caused the incident.

Yoshie said the company had not expected an undercarriage to get a crack. He said all undercarriages will undergo checks by the end of January.

On Dec. 12, the safety board recognized the case as the first "serious incident" affecting the shinkansen system since 2001 when the board's predecessor organization was established. A serious incident is defined as a situation deemed to bear the risk of causing an accident.