A cleaner on her way to the night shift at a Toronto-area university was killed in a GO bus rollover, the first passenger to die in the transit agency's nearly 50-year history.

Police are trying to determine why the bus crashed in clear and dry conditions with no other vehicles involved. GO Transit has launched its own investigation and was tight-lipped on Thursday about the incident.

The eastbound bus struck the guardrail on Highway 407 and rolled off the road late on Wednesday in Vaughan. It came to a rest on its side with a long piece of the guardrail inside. Police identified the victim as Radika Nankissoor, 56, of Brampton, Ont. She was ejected from the bus, pinned underneath and died at the scene, police said.

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The crash on route 47, which begins in Hamilton, happened only minutes from the bus's final destination, York University.

In a statement, York called Ms. Nankissoor "a valued member of [our] staff." According to a university spokeswoman, the victim had been a custodian at York since 2010 and had been scheduled to work on Wednesday night.

GO Transit president Greg Percy said on Thursday it was the first time a GO passenger had been killed since the agency was formed in 1967.

"Our thoughts and prayers are with the families involved," he told a news conference. "Our prime commitment has always been and will continue to be to keep our passengers safe."

Mr. Percy said the bus driver was "experienced," with more than 15 years behind the wheel. He would not talk about the medical history of the driver, saying that would be part of the police investigation.

Anne Marie Aikins, a spokeswoman for Metrolinx, the regional transit agency that includes GO, said an internal investigation would include "all aspects" of the incident, including driver training and vehicle inspections. She would not discuss how long the driver had been on duty.

Under the collective agreement, drivers are limited to shifts no longer than 12 hours and 35 minutes, according to Metrolinx. There are mandated breaks, and many drivers work split shifts to cover the morning and evening rush hours.

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Ms. Aikins said buses have limiters that restrict speed to 109 kilometres per hour. They do not have technology that can help detect when drivers are getting drowsy.

Police said the bus struck the guardrail near the 407 and Weston Road "for unknown reasons" and then rolled off the highway. The investigation closed the eastbound highway for hours.

The other four passengers were in their late teens or early 20s. Two had minor injuries, as did the 66-year-old driver.

OPP spokesman Sergeant Kerry Schmidt said on Thursday morning that police talked to the people aboard and that passengers had no sense anything was amiss.

"There was nothing specific that led them to believe there was going to be an impending collision," he said, adding. "No explosion … no big bang."

Late the previous evening, a police spokesman on the scene about an hour after the crash said the site was dry and clear. There were "no environmental conditions that would factor into this," he said.

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Police were looking at the driver's actions and the condition of the bus. "Hopefully, the mechanical examination will give us an idea what the vehicle was doing before the collision," Sgt. Schmidt said.

According to Mr. Percy, the GO Transit president, the bus had an onboard computer that would show, among other things, how fast the vehicle was going. No cameras were on board. He said the bus was eight years old and "mechanically was fine."