A shinkansen made an emergency stop on Friday inside the nearly 54-kilometer undersea tunnel on the newly opened Hokkaido Shinkansen Line, the railway operator said Wednesday.

The driver stopped the train because an emergency signal was triggered by a small metal object lying near the track, Hokkaido Railway Co. officials said. It was the first emergency halt by a bullet train since shinkansen services began between Tokyo and Hokkaido on March 26.

The revelation of the trouble threatens to dampen enthusiasm for the new line soon after it opened to great fanfare.