The El Cerrito del Norte BART station reopened and is operating on normal Sunday schedule after it was shut down due to the incident, according to the transit agency.
Agency officials told ABC7 News the stabbing happened on the station platform at 10:20 p.m.
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The victim was airlifted to John Muir Medical Center in critical condition.
Mutual aid had to be called in because unruly passengers were getting in the way of the investigation.
BART opened a bus bridge for passengers from the El Cerrito station to the Richmond station.
Bay City News contributed to this story.