Slippery Rail Season is one of the most dreaded times of year for any rail-based transit system; however, it’s one of the least understood phenomenons by the public and is often heralded as a lame excuse for poor service.

Here’s what happens. During the autumn season, leaves fall onto the tracks and are subsequently crushed by passing trains. The crushed leaves then create a thin film of what’s best described as a slippery wax. This, in turn, creates conditions akin to black ice: the film is invisible to passing trains, and when they attempt to stop, they skid and slide until enough traction and friction brings them to a halt.

Railroads constantly attempt to combat this by trimming trees in advance of autumn and operating special high-power water jets along the tracks to rid the rails of the thin film. However, you can only clear so much, there’s a limited number of power washing trains, and running a power washing train means a train with customers can’t operate along that track segment at the given time (well, they could, but they’d have to operate at such a slow speed you wouldn’t want to).

So, the next time you’re stuck on a slow-moving train in the fall or you overshoot the platform at your station, you know why.