Homicide detectives are investigating after a man died in the custody of train guards at a Melbourne train station last night.

A commuter on a train at Merri railway station "sought assistance from Protective Service Officers" (PSOs), police said.

The man who appeared substance-affected got off the train, and PSOs remained with him.

PSOs called an ambulance, and handcuffs were placed on the man due to his increasingly erratic behaviour.

At around 9.45pm, the man became unresponsive, and the PSOs commenced CPR.

The 63-year-old man couldn't be revived.

Detectives from the Homicide Squad will now investigate the incident with oversight from Professional Standards Command, as per standard practice when a person dies whilst in police custody.

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