A portable phone charger apparently caught fire inside a college student's backpack Monday while he was riding Tokyo's busy Yamanote Line, police said.
Smoke started coming out of the backpack when the train was traveling from Tokyo Station to Kanda Station at around 1:10 p.m.
The student got off the train at Kanda, where station employees doused the backpack with a fire extinguisher.
No one was injured, public broadcaster NHK reported.
The police said they found the burned battery, used to charge smartphones and other handheld devices, in the backpack. They opened an investigation to determine what caused the battery to catch on fire.
East Japan Railway Co., known as JR East, said the incident delayed 17 trains for up to 20 minutes, affecting an estimated 11,000 people.