Toronto's first big heat wave of the season has begun, and while the temperatures are high enough to have everyone blasting their air conditioning and seeking shade, they're also so extreme that it's impacting key infrastructure and services.

Metrolinx issued a news release Tuesday afternoon that certain transit routes are experiencing disruptions as a result of today's high of 33 C, which feels closer to 40 C with the humidex, resulting in a heat warning and special air quality statement.

"During prolonged periods of high heat, rail tracks have the potential to soften and expand that require us to run trains at slower speeds. To ensure the safety of our customers and crew, slow orders are issued along the network which also reduce the risk of track damage," the agency wrote.

Impacted lines include the Stouffville GO, Lakeshore East GO, Lakeshore West GO and Kitchener GO trains.

Some trips have been completely cancelled, including the 3:20 p.m. train from Union Station to Durham College Oshawa GO and the 4:40 p.m. train in the other direction. Other schedules have been adjusted to later times (like the 3:12 p.m. departure from Union Station to Mount Joy), or replaced with GO buses (some of this afternoon's Stouffville Line trips).

And, still others are running at slower speeds due to the acute heat, with select Lakeshore West and Kitchener trains arriving 5-10 minutes later than planned this afternoon and evening.

"We're adjusting the arrival time for all outbound express Lakeshore West trips from Union Station between 3:30 p.m. and 6:00 p.m.," a service update on the GO website reads regarding the former route.

"If you normally take an express westbound train in that timeframe, you will now arrive at your destination approximately five minutes later than previously scheduled as we operate at slower speeds due to extreme heat. We do not expect these changes will impact your departure time; we expect your trip will still depart Union Station at its usual time."

Thankfully, these disruptions will be temporary, with changes set to affect routes until 8 p.m. at the latest. Then again, at the time of publication, Wednesday's forecast for Toronto looks to be almost as scorching, though some potential thunderstorms could cool things off.