From CNN:

"There was a bloodcurdling scream. I was just reading my book, and all of a sudden, I heard it," Garnet Caton, who was sitting in front of the two men, said of the Wednesday night incident west of Portage la Prairie in Manitoba.

"It was like something between a dog howling and a baby crying, I guess you could say," Caton said. "I don't think it will leave me for a while."

from The Guardian:

It came out of nowhere, passengers on the Canadian Greyhound bus said. A young man was sleeping, head against the window, when the man sitting next to him began stabbing and then decapitating him as other travellers looked on in horror....

When he was attacking him, he was calm - it was like he was at the beach," added Caton, who was sitting in front of the victim. "There was no rage or anything. He was just like a robot stabbing the guy." ...

Caton and the driver soon returned to the vehicle, however, and found the assailant still hacking the young man's body into pieces.

The severed head of the victim, described as in his 20s, was then displayed to the stunned passengers.

From Yahoo! news:

Caton and the driver shut the bus door from the outside while they waited for police to arrive.

"We put our bodies up against the door, waiting for him to come out ... and he went back and brought the head to the front and pretty much displayed it ... and dropped it on the ground in front of us," Caton said.

"All very calmly. He was wearing sunglasses. It was no big deal to him."

"The guy came to the front of the door with buddy's head in his hands, decapitated. He dropped the head and went back and started cutting the body back up," Olmstead said.

The victim and attacker never spoke to each other, Caton said, as the victim was listening to music through headphones...Passengers had no explanation either as to what might have prompted the attack.