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Temperature spike recorded in Shinkansen undercarriage before crack discovered

In this file photo, the 13th carriage of a JR West N700 series "Nozomi No. 34" bullet train, where a crack and oil leak were discovered, is shown at JR Nagoya Station on Dec. 12, 2017. (Mainichi)

The temperature of a Shinkansen bullet train car undercarriage found to be cracked rose by 10 degrees Celsius on part of its run from Tokyo to Fukuoka on Dec. 11 last year, before it was forced out of service on the return journey.

During a roughly 200-kilometer stretch between Kanagawa and Aichi prefectures, the temperature of the undercarriage of the No. 13 car, where a crack was later discovered, was 15 degrees Celsius higher than the other car undercarriages. Nearly four and half hours before the problems surfaced as strange smells and noises, problems with the undercarriage had apparently already developed.

According to Central Japan Railway Co., infrared sensors in Odawara, Kanagawa Prefecture, and Toyohashi, Aichi Prefecture, recorded a temperature rise in the joint transferring power from the motor to the train's wheels. The increase was within standard limits, so a warning was not issued, and the unusual conditions were only discovered during an investigation following the discovery of the undercarriage crack. The exact figures have not been made public.

However, internal documents obtained by the Mainichi Shimbun show that the recorded temperature of undercarriage in question was roughly 30 degrees Celsius in Odawara, much higher than the train's other undercarriages. By the time the train had reached Toyohashi, the temperature difference was even more extreme, with the temperature nearing 40 degrees Celsius.

There are no infrared sensors along the 800 kilometers of track from Toyohashi to JR Hakata Station in Fukuoka, so no further temperature information exists. However, it is believed that the bend in the undercarriage frame due to the crack deformed the motor joint, which then heated up as it spun at high speed.

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