Kawasaki Heavy Industries Ltd. excessively ground down a steel component in 2007, likely causing the 16-cm crack that was discovered in December on the undercarriage of a shinkansen, West Japan Railway Co. said Wednesday.

The thinnest part of the component was 4.7 mm, 3.3 mm less than designed, the railway operator said.

JR West found 100 shinkansen undercarriages with similarly over-grounded parts. The operator, however, has found no immediate safety problems because it has confirmed that there were no cracks in the parts through tests using ultrasound monitoring devices.

JR West said the undercarriages have maintained enough strength to support the train cars but that it will gradually replace the problematic parts within a year. It will continue to monitor the parts’ strength and ensure safety until they are replaced, the company said.

Yoshinori Kanehana, president of Kawasaki Heavy, apologized at news conference in Kobe later in the day.

“We have caused much trouble and anxiety. We deeply apologize,” Kanehana was quoted as saying by Kyodo News.

The Dec. 11 discovery of the damaged part on Nozomi 34, a Tokyo-bound shinkansen, caused a public outcry, with the operator admitting it could have led to a major crash.

The train’s conductor suspected there was an issue with the train due to the presence of an odor, initially detected while the shinkansen was traveling near Kokura Station in Fukuoka Prefecture.

But the train continued for another three hours and the crack was found only after it reached Nagoya Station. Oil had dripped from the undercarriage in question.

JR West later said the crack could have been serious enough to cause a derailment.

According to the railway company, Kawasaki Heavy ground down the bottom part of the component in question to adjust the joint part with another component. This apparently increased stress to a welded area and caused the crack, a JR West spokesman told The Japan Times by phone.

According to a press release posted on JR West’s website, Kawasaki Heavy admitted to the railway operator that this “was something that shouldn’t have been done.”

According to Kyodo News, Central Japan Railway Co. (JR Tokai) said the same day it has 46 undercarriages with the same components and plans to replace them by the end of December.

Kyushu Railway Co. said none of its undercarriages are affected by the problem, according to Kyodo.

JR Tokai operates shinkansen services between Tokyo and Shin-Osaka, while JR West runs bullet trains between Shin-Osaka and Hakata, Fukuoka Prefecture.

The discovery of the crack significantly damaged the safety reputation of the shinkansen system, which hasn’t had a fatal accident since launching in 1964.

On Dec. 12, the transport ministry’s accident investigation board determined the incident to be the very first “serious incident” affecting the high-speed train system since 2001, when the board was established.

A serious incident is defined as a situation with the potential to cause an accident

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