An unbuckled seat belt set off a series of events that led to a SEPTA bus careening out of control, jumping a concrete barrier and landing on train tracks in South Philadelphia, the transit authority said. The crash injured the bus driver and two passengers.

The Route 29 bus driver was making a U-turn along Columbus Boulevard at Dickinson Street around 6 p.m. Tuesday when she slipped out of her seat and lost control of the bus.

SEPTA spokeswoman Jerri Williams said the driver was trying to detour around a CSX freight train that blocked the bus' usual path at Tasker Street. Bus drivers are required to wear seat belts at all times, Williams said.

"We believe that this accident would not have occurred if she had her seat belt on," Williams said.

Surveillance video from onboard the bus showed the driver, a 10-year veteran, falling from her seat and the violent crash. Two women passengers were thrown forward from the impact. All three were hospitalized and suffered non-life-threatening injuries. A third passenger was not hurt.

CSX workers on the train, stopped just feet from the crash, immediately rushed to render aid and call 911.

Witness Lauren Ferrett told NBC10 on Tuesday it appeared the bus took the U-turn at a high rate of speed.

SEPTA continues to investigate the incident. The driver will face disciplinary action and could lose her job, Williams said.