Terrell Coffer, 44, of Oakland allegedly stabbed the victim aboard a train as it approached the Warm Springs/South Fremont Station shortly before noon Friday.
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A BART community service officer on the platform spotted Coffer and helped detain him. Coffer is being held at Santa Rita Jail on suspicion of attempted homicide, according to BART police.
Authorities have not released information about what led to the stabbing, but said it doesn't appear that Coffer and the victim knew each other.
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