A broken rail delayed subway trains on the Tokyo Metropolitan Ginza Line for 4 hours and 40 minutes Monday, stranding some 99,000 commuters.
The broken rail was discovered near Shibuya Station at around 8 a.m., canceling subway runs between Shibuya and Tameike-Sanno stations. One train got stuck between Omotesando and Shibuya stations for an hour.
Tokyo Metro said one of the two tracks between Shibuya Station and its train garage, which is used for route changes, was found damaged.
The tracks, however, are not used to carry passengers, metro officials said, noting that out-of-service cars pass by once every several minutes.
Subway staff spotted the broken rail during an inspection triggered by an alarm that indicated a malfunction at the switching point.
Tokyo Metro said no track failures had been detected during the ultrasound exam on April 18 and that it is investigating the cause.
The Ginza Line section between Asakusa and Tameike-Sanno stations was unaffected by the problem.