East Japan Railway Company says Tuesday's extensive suspension of Shinkansen bullet train services was caused by damage to a part necessary to keep the overhead power cable in place.
A power outage in the morning led to the cancelation of 283 trains on one part of Tohoku, Hokuriku and Joetsu Shinkansen lines for the rest of the day and affected about 120,000 people.
The operator said on Wednesday that workers found an overhead power cable dangling at a location between Omiya and Tokyo's Ueno stations. They found a fracture in the metal rod that supports a 1.3-ton weight designed to adjust the tension of the cable.
JR East said the Kagayaki bullet train on the Hokuriku line touched the sagging cable when it passed through, triggering a power outage when the system detected an anomaly. The train that came to a stop near the site sustained damage to its pantographs and one of its windows.
The operator says that under the company's standard maintenance policy, the replacement cycle for this type of part is every 30 years or so. But it says actual replacement is carried out for each part depending on its condition, and the part that broke had been in use for 38 years.
JR East says no abnormality was reported for the part in visual checkups conducted in June and October last year.
It says it will carry out emergency checkups this month at about 500 locations along Shinkansen lines where the same kind of part is used. An employee in charge said the company plans to ask experts to examine the cause of the fracture, and discuss how to prevent a recurrence.