The incident took place on Tuesday night aboard a Churchgate-Nallasopara fast local train.

The victim, identified as Mayank Lohar, was travelling in the first-class compartment when he got into an argument with another passenger, later identified as Roshan Suvarna.

Police said the accused was arrested from Mumbai's Kurla area shortly after the incident.

Argument turns deadly

According to the Western Railway Police, the altercation began between Goregaon and Kandivali stations over whether the train door should remain open amid heavy rain.

As the argument intensified, some passengers allegedly assaulted the accused. Enraged, Suvarna reportedly pulled out a knife from his bag and attacked Lohar.

"Between Goregaon and Kandivali, two passengers fought over closing the train's door amid rainfall. During this time, the accused stabbed the victim with a knife. He then fled the scene," Datta Khuprekar, Senior Police Inspector of Borivali Railway Police, said.

Lohar sustained fatal injuries and died on the spot, police said.

Chilling video captures aftermath

A disturbing video recorded inside the train showed the accused, dressed in black and carrying a knife, walking away after the attack.

Bloodstains were visible on the metal seats of the compartment as shocked passengers moved away from the scene.

"Ye mar diya usko, mar diya (He killed him—he actually killed him)," people could be heard saying in the video.

The footage also showed the accused turning back to look at the victim, who lay motionless in a pool of blood on the train floor.

Family demands justice

The victim's family expressed grief and anger over the killing, demanding strict punishment for the accused.

"Usko saja do, mere bete ko nyaya do. Woh kisi se na ladta tha, na jhagadta tha. Kal ka gaya hai, abhi tak aaya nahi mera beta. Kidhar jaaun, barbaad ho gayi," Mayank's mother told reporters through tears.

(Punish him. Give justice to my son. He never fought or quarrelled with anyone. He left yesterday and never returned. Where do I go now? I am ruined.)

His sister demanded the harshest punishment for the accused and questioned why nobody intervened during the attack.

"It was 10:30 at night. Where were the people? The knife was huge. I want justice. He should be sentenced to death. If he isn't punished today, someone else will die tomorrow," she said.

"My brother never caused trouble and never got into fights. He kept to himself and had his whole life ahead of him," she added.

Investigation underway

Railway Police reached the scene soon after the incident, took custody of the body and sent it for post-mortem examination.

Police have registered a case and are investigating the circumstances that led to the fatal stabbing. Further legal proceedings against the accused are underway.