Needles allegedly found in a train seat on Tuesday, January 8, 2019.

A teenager accused of planting 20 needles on a Melbourne train seat, puncturing an unsuspecting passenger's posterior, was on bail when he allegedly committed the offence.

The 16-year-old boy, who cannot be named for legal reasons, faced a Children's Court on Tuesday on charges including intentionally causing injury, assault with a weapon and committing an indictable offence while on bail.

He also faces charges of recklessly causing injury and unlawful assault.

Supported by his mother and grandfather, the teen was not required to speak in court.