It happened just before 8:30 a.m. Thursday at Boston Road and East Tremont Avenue.
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Surveillance video showed the moment of impact, which smashed the windshield, and video from NewsCopter 7 and from the Citizen app showed the front end of the B21 bus in contact with the pillar.
The exact cause of the crash has yet to be determined but so far investigators say it appears the driver slumped over and went into cardiac arrest moments before the crash.
The 44-year-old bus driver was taken to St. Barnabas Hospital in critical condition, after being treated by EMTs for cardiac arrest.
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Twelve passengers, including a number of children, were taken to area hospitals for injuries described as non-life-threatening.
Eyewitness News spoke to the mom of a 12-year-old girl who taking the bus on her way to school.
That mom came to the scene shaken, knowing her daughter was there, and learned the girl was being transported to the hospital.
"I was worried," she said. "I wasn't mad, I was worried... She goes to summer school, she takes it every morning. First time this happened."
Officials said the driver is a 14-year veteran of the MTA with a clean driving record, and that her family was at her side at the hospital.
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Her sister is also an MTA bus driver.
The driver passed her bi-annual physical, as required of all MTA bus operators, and that she checked in for work as usual Thursday morning with no indication of any health issues.
Officials said there was no structural damage to elevated train pillar or tracks.
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