Toronto police say the unidentified rider was in the saddle this morning when the horse went into an "unexplained wild gallop."
The trainer was crushed into the track railing and then by the weight of the horse as it collapsed.
A horseracing source says the horse, Tawney's Wish, was a three-year-old dark filly with one start this year and $368 in winnings.
Officials from the provincial Ministry of Labour are on the scene, along with police forensics experts and detectives.
The identity of the trainer has not been released.