Eyewitnesses told national broadcaster NHK of their fear during the attack that took place on Halloween when the Japanese capital was full of revellers, many in costume.
Media outlets said the alleged 24-year-old perpetrator - wearing a green shirt and indigo suit similar to that worn by the character Joker in the Batman franchise - attacked people with a knife and started a fire on the train.
Kyodo News and NHK reported that 17 people were wounded, with one elderly man in critical condition after being stabbed.
A video clip published by NHK showed panicked passengers running through the train as smoke filled the carriage, with flames also visible.
Another video posted on Twitter showed people climbing through windows to escape the Keio Line train at a station.
The line operator said services had been partially suspended after “an incident involving injuries” took place just before 8:00pm near Kokuryo in the Japanese capital’s western suburbs.
Dozens of firefighters and police officers were seen working outside the station.
“At first I thought it was something like a Halloween event. But I rushed away as a man carrying a long knife came in. I was very fortunate not to be injured,” a man who was on the train told NHK.
The attacker carried out the act without showing any emotion, a female passenger said.
“He held a knife and started spreading liquid,” she said.
“He was committing this act without showing any emotion, just mechanically. I think that brought fear to everyone.”
Kyodo reported that the suspect had told investigators he “wanted to kill people and be given the death penalty”. He also told them he had spread lighter fluid in the train.
Local police officials could not immediately be reached for comment. Regional police said they had no information for immediate release.
Rare violent crime
Violent crime is rare in Japan, but in August, nine people were wounded, one of them seriously, in a stabbing attack on a commuter train in Tokyo, with the suspect later handing himself in after fleeing the scene.
In a separate attack that same month, two people suffered burns in an acid attack at a Tokyo subway station.
Japan has strict gun laws, but there are occasional violent crimes involving other weapons.
In 2019, a man killed two people including a schoolgirl and wounded more than a dozen in a rampage that targeted children as they waited for a bus.
And in 2018, a man was arrested in central Japan after stabbing one person to death and injuring two others aboard a bullet train.