On the night of Friday October 7, 1966, 42 teenagers from the Cité des Jeunes high school in Vaudreuil were heading to a dance in nearby Hudson.
But they never made it.
While en route, their bus stopped at a level crossing on St-Charles Avenue in what was then Dorion. The gates were down. An eastbound passenger train passed through, then the bus starting moving again, and a freight train heading in the opposite direction plowed into it. The bus was split in two, with the front part being dragged hundreds of metres along the tracks.
19 people died almost instantly from the impact, with one more boy dying later on hospital from his injuries. The 1998 death of one of the victims, Nicole Bélanger, was also attributed to the crash as she never fully recovered from her injuries.
After multiple inquests and inquiries, nobody was ever found responsible. An explanation for why the crossing gates were raised has also never been given, and the level crossing has since been converted to an underpass.
Vaudreuil-Dorion mayor Guy Pilon was a student at the Cité des Jeunes high school at the same time as the accident, and remembers the mass funeral a few days after the accident.
"I remember the coffins in the gym at Cite-des-Jeunes, and the prime minister, the ministers were there. A lot of black limousines, I still have this picture in my head."
He said the accident is still seared in some locals' minds as a major tragedy.
"Even today you don't see the name Vaudreuil-Dorion in newspapers often. Imagine at that time, Dorion? It was the front page of every newspaper in Quebec, in Canada, it was worldwide."
Pilon said some of those who survived and those who were close to victims are still dealing with the pain now, 50 years later.
"For the ones who were there, it's very sensitive. People don't want to talk too much about it, they have their memories."
A memorial service for the anniversary will be held Friday night at Vaudreuil-Dorion's Trinity Church, and church bells will sound at 7:35 p.m., the time of the accident.
Pilon said he will be there to offer emotional support.
"I will not be there as a mayor, I will be there as a friend and someone who knows people who were there at the time."