The maker of a shinkansen undercarriage on which severe cracks were found last year offered an apology Friday over the incident, which could have caused a derailment, citing a vulnerability in its quality management system.
"We have relied too heavily on the judgment and discretion of employees at the (manufacturing) sites" in terms of quality control, Yoshinori Kanehana, president of Kawasaki Heavy Industries Ltd., said at a news conference in Kobe.
Last December, the affected bullet train operated for more than three hours — with a burning smell and an abnormal motor sound — for a distance of 800 kilometers after it left Hakata Station in Fukuoka bound for Tokyo.
The Japan Transport Safety Board designated it as the country's first serious incident affecting the high-speed train system.
An investigation discovered a 16-centimeter crack at the bottom of a steel frame holding the set of wheels and two 14-cm fissures on both sides of the frame.
In February, the manufacturer disclosed it had shaved the steel frame thinner than the minimum required for the component when it produced the undercarriage, a practice that violated the company's production manual. Kawasaki Heavy said workers did not fully understand the document.
Workers scraped off the bottom of the undercarriage frame to smooth the surface before welding work, the company said.
"The ban on shaving of the steel frame was not explained to workers at the manufacturing site as there was a lack of communication," said Munenori Ishikawa, a Kawasaki Heavy vice president.
The expansion of the cracks had affected the strength of the undercarriage parts a day before they were discovered, according to the safety board investigations.