(CNN) -- At least three people were killed Sunday when a passenger train derailed near Toronto, Canadian officials said. All three people were employees of Via Rail Canada, Michelle Lamarche, a spokeswoman for the railway, said, calling it a "very sad day for Via." At least 45 passengers were injured, three severely, according to John Marginson, chief operating officer of Via. The derailment occurred around 3:30 p.m. in Burlington, about 40 miles southwest of Toronto, Lamarche said. The train, carrying 75 passengers, was traveling from Niagara Falls to Toronto, according to Lamarche. An investigation into the cause of the wreck is under way.
NEW: 45 passengers are injured, rail line official says . Those killed were employees of Via Rail Canada . The train was carrying 75 passengers .