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The chance of a train going out of service increases when the weather spikes, Ross said, because of a greater strain on the HVAC systems to keep trains cool. This August was the hottest one on record in Toronto, with temperatures rising 20 times above 29 C. In August, there were more cases of hot cars forcing trains out of service — 25 — than in the entire four-month span between June and September in 2015. Five cars were taken off the line because they were too hot on Aug. 12, when the temperature spiked to a high of 35.9 C.

But the TTC continues to place trains out of service due to hot cars, even when temperatures dip in the fall. A train on Line 2 was put out of service on Oct. 9, 2015 because it was too hot, despite temperatures only reaching 16.7 C that day. Later, on Nov. 5, 2015 — Toronto saw a high of 20.5 C — a train was taken out of service again because of hot cars.

When the trains are taken out of service, riders are completely offloaded and their trips are delayed for an average of three minutes. The train’s operator is then able run the train privately until it arrives at a carhouse where it can be repaired.

Each of the TTC’s 18-to-20-year-old, six-car T1 trains that run on the Bloor-Danforth line have an air-conditioning unit implanted into the bottom of each car, while evaporators and fans sit on the roof. The decision to place a train out of service isn’t made when temperatures cross a certain threshold. Trains are only put out of service when temperatures swell in three of the six cars, or when the front or back cars lose cooling. The front cars carry train operators and the back cars carry guards who are tasked with operating the train’s doors — and these employees can’t work in “saunas,” Ross said.