Metrolinx officials say a citizen calling 911 may have prevented a "potential disaster" during the evening commute on Wednesday.
The caller told police that a construction forklift was blocking train tracks near Guelph.
There was a full GO train on its way to Kitchener at the time, with about 1,800 people at the time.
Metrolinx spokesperson Anne-Marie Aikins says that if the train had hit the forklift, it is possible it could have derailed.
The train was stopped just in time, about 200m away from the vehicle. Officials say it takes about 1km for a train to come to a stop.
Police are now investigating why the vehicle was on the tracks, but it appears it rolled onto the tracks from a nearby construction site.
"Some speculation at this point... but it clearly was not supposed to be there, nor did the construction crew want it to be there," Aikins says. "It seems to have inadvertantly got there."
There was only a 15-minute delay while the forklift was moved, and the train moved on.