Today I learned that fallen leaves can gum up Toronto's many outdoor subway lines so much so that operators have to stop or slow their trains down to a crawl.

Autumn problems, am I right?

As reported late last month, a recent TTC investigation found that parts of the wheels on some Line 2 subway trains are rubbed down into "flats," causing loud vibrations as they move through the city.

Debris on the tracks, such as fallen leaves, can cause these bald spots. Wet leaves, in particular, can also make the top of the rail slippery, further grinding down wheels.

Excessive rail lubrication in west end, plus wet rail and leaves, has resulted in wheels being overly slick, which causes spinning, which causes the speed control system on the train to think it has lost control, resulting in emergency braking then skidding, which can cause flats

— Brad Ross (@bradTTC) October 30, 2018

It makes sense that operators would want to prevent this or any other kind of potential malfunction, but it's still kind of funny to hear "leaves on the track" as a reason for slow service.

"The Southbound train to Bloor-Yonge is stopped because of fallen leaves on the track?" wrote one passenger on Twitter Tuesday evening.

"The announcement from the operator is trains are moving slowly (very slowly) due to fallen leaves between Bloor and Rosedale," he wrote in another tweet to the TTC Customer Service Account. "You have got to figure that out."

The account replied to confirm that, sometimes, "trains may move slowly due to fallen leaves as it makes it more likely for our train wheels to slide when attempting brake application." 

Oh it stopped. At Rosedale. For a while. Then crawled towards Bloor. For fallen leaves.

— !PO! (@hungrypo) November 7, 2018

It might sound silly to stop for leaves—those things we jump into piles of and crunch underfoot every fall—but huge amounts of anything can turn into an obstacle.

So, the next time you're stalled near an outdoor subway track, you can blame your colourful autumn Instagram fodder.

Are the leaves pretty? Sure, but they're dang pesky too.