BOSTON (CBS) – For some commuters, the heat is more than an aggravation – it means transportation delays.

“A little more irritated than usual … because the heat has a way of making you frustrated,” said one commuter.

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And extreme heat, slow trains and a crowded commute can lead to a lot of frustration. And this July has been Boston’s hottest to date.

“Due to extreme temperatures forecast for today and tomorrow, trains may travel slowly in some areas to reduce heat-related stress on the tracks,” the MBTA tweeted Tuesday.

What is this “heat-related stress” that’s making you late for work?

According to Keolis, which operates the commuter rail, the reason for the heat-related speed restriction is to prevent train derailments. The steel tracks expand in hot weather and shrink in cold weather, so ties on the tracks keep them from shifting. However, the combination of very hot weather and vibrations from trains means the rails can buck their ties and shift suddenly – what is known as a “heat kink.” These kinks – a curve in the rails or sudden movement in the track – can cause train derailments.

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“Speed restrictions mitigate the risk to operate over a heat kink as it gives more time for the engineer to react and it will also limit the heat added to the tracks caused by the friction of the speeding train,” the MBTA said.

The MBTA will have crews performing track inspections during the heatwave to look for problems on the tracks.

Amtrak has reported at least one heat-stressed track in the Forest Hills area that had to be closed for repairs.

Wires that power some trains, trolleys and buses are another concern. These wires can expand and sag in the heat, so MBTA crews were working to prevent low-hanging wires.

One rider said the delay added about five minutes to her commute.

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“It wasn’t affecting my line that much – maybe by five minutes or so, but not too much,” said Mary Tseng, of West Roxbury.