Environment Canada said some parts of the city had been drenched with more than 3.9 inches (10 centimetres) of rain in yesterday's evening storm, easily beating the previous one-day rainfall record of 1.4 inches (3.6 centimetres) in 2008.
Toronto police and firefighters used boats to rescue commuters from a 10-car, double-decker train that stalled in floodwaters that reached up to the lower windows. Murky brown water spilled through the bottom floor of the carriages, sending passengers fleeing to the upper decks.
"There's a full-on river on either side of us... We. Are. Stuck. Hard," passenger Jonah Cait wrote on Twitter.
Another passenger told the TV news network CP24 that she could see people clinging to trees after abandoning their cars on a flooded highway alongside the tracks.
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Metrolinx said the Toronto police marine unit floated out in rescue boats to pull weary passengers through the windows about three and a-half hours after the train got stuck.
All of Toronto's subway service was temporarily halted due to power and signal issues. Some stations were also flooded. Partial service later resumed but large parts of the system were still shut down.
The storm left the downtown core dotted with abandoned vehicles, some sitting in water up to their windows. One woman, in a T-shirt and shorts, dove head-first through the window of her marooned car before wading away in the thigh-deep currents.
As many as 3,00,000 Toronto Hydro customers lost power. Hydro spokeswoman Tanya Bruckmueller said efforts to restore power to customers might be slowed as night fell.
Another utility, Enersource, said power was cut to about 80 per cent of Mississauga, a suburb of 700,000 west of Toronto. By around 10 pm, local time only about 50,000 were without power.
Toronto's flash flooding comes two weeks after extensive flooding in Calgary turned parts of the western Canadian city into a lake and forced up to 1,00,000 Albertans from their homes. Three bodies were recovered during the floods.