Cow slaughter is banned in most Indian states whose predominantly Hindu population consider the animal to be sacred.
But Hindu hardliners and cow vigilante groups have taken to enforcing the law themselves ever since Prime Minister Narendra Modi's Hindu nationalist government came to power in 2014.
On Thursday evening 16-year-old Junaid Khan was killed and three others were injured in a fight that erupted in a train soon after it left Delhi for the northern town of Mathura.
Police officer Kamal Deep Goyal said the fight started over an argument over seats in the train. A group of more than a dozen men accused Junaid and his family, who were returning home after shopping for Eid, of having beef in their bags.
"They started abusing us, saying we were Muslims and traitors. They said we should go back to Pakistan," Junaid's brother Shakir told television channel CNN-News18.
Goyal said one of the attackers had been arrested and they were hunting for others in the group.
The attack was the latest against Muslims who make up about 14 percent of India's 1.3 billion population.
In April, a Muslim dairy farmer was lynched in the north-western state of Rajasthan for allegedly smuggling cows.
Opposition groups have criticised Modi for failing to speak up against the violence targeted at Muslims, accusing his party of a sectarian bias.
"An atmosphere of hatred and Islamophobia is being created in the country," said Asaduddin Owaisi, the head of the All India Majlis-e-Ittehadul Muslimeen (All India Council of the Union of Muslims).
Shakir, brother of the killed teenager Junaid, at a Delhi hospital. Shakir was stabbed five times.
Indian police have arrested a man for allegedly stabbing a Muslim teenager to death on suspicion of possessing beef, an officer said on Sunday, an attack that has drawn anger across the country.
