Giving the Train a Slip

Every autumn, leaves fall from trees. If you’re a homeowner, it can be a chore to clean up — raking leaves can take a long time. And when they get wet, forget it — it’s a big old mess. That’s the bad news. The good news is that wet leaves, while kind of gross, are typically harmless. Sure, they can make for slippery conditions, but you can get around that by simply walking where the leaves aren’t. 

For mass transit, though, wet leaves can be a major issue. When these leaves stick to the rails, they create a slippery condition that can cause havoc as trains accelerate and decelerate. In the United States, this is often called “slip-slide conditions” and in the UK, the problem is often articulated as simply “leaves on the line.” Either way, this loss of adhesion leads to delays. For example, in October 2020, authorities at Metro-North, a Manhattan-bound commuter rail line, warned that “gelatinous slime” from crushed leaves can “automatically make an emergency stop, because the on-board computer system perceives ‘slip-sliding’ as the train not slowing down when it should.” 

And that can be bad. For example, in September of 2013, the Joondalup railway line in Western Australia had a crash caused by slip-slide conditions. A train traveling from Perth couldn’t slow down in time as it entered Clarkson station; something was preventing the train from getting the traction it needed. That train rear-ended into a parked train, causing confusion and delay. (Thankfully, the crash didn’t cause many injuries; per NBC News, “six passengers were treated for stiff necks” but no one was seriously hurt.)

But if you look at the timing of the crash, you’ll immediately notice that leaves weren’t to blame — September isn’t the start of fall in the southern hemisphere; it’s the start of spring. The explanation? Leaves aren’t the only things that can leave a gelatinous slime on the tracks. So can these guys.

That, pictured above, is the Portuguese millipede. As the creature’s name suggests, it is indigenous to Portugal (and the Iberian peninsula generally). Back home, the Portuguese millipede has a number of predators which keep its population in check. But in the 1950s, the bug was accidentally introduced into parts of Australia. And in that region of the world, the Portuguese millipede doesn’t have any natural enemies. For decades — peaking in the 1970s or 1980s — the Australian population of Portuguese millipedes grew and grew. Efforts by the Australian government to keep the critter’s population in check have been mostly successful, but at times, a confluence of factors can cause a momentary explosion in the number of Portuguese millipedes in different parts of the nation.

And, like leaves, Portuguese millipedes don’t really know what train tracks are. As Reuters reported, when investigators went to inspect the tracks after the Clarkson crash, they discovered “hundreds of the tiny creatures [. . .] squashed in a slippery mess on the track.” The Atlantic explains what probably happened:

The creatures [Portuguese millipedes] have a tendency to hang out on train tracks, it seems, their shiny-black exoskeletons acting as perfect camouflage. And when a train comes along, they … well, you know. And when there are a lot of millipedes being squished at the same time, that leads to tracks that are much less friction-filled than normal. “The train loses traction and the train has slipped,” explained David Hynes, a spokesman for the Public Transport Authority of Western Australia.

This wasn’t the first time the millipedes proved to be a problem for Australian railways; as Time reported, in 2002, “so many of the Portuguese millipedes clogged the rails between Melbourne and Ballarat — about 70 miles apart — that 50 trains had to be suspended.” Similarly, in 2009, according to Western Australia Today, “a plague of the millipedes overran railway tracks at Tallarook in central Victoria, causing several trains to be canceled.”  And it probably won’t be the last. But authorities aren’t too worried — it’s a rare occurrence and, as those of us impacted by leaf-caused slip-slide conditions know, train technology can usually keep the people on board safe. The same, however, cannot be said for the millipedes.

Bonus fact: Millipedes aren’t insects; both millipedes and insects are from the arthropod phylum. But millipedes may be a decent insect repellent — especially if you’re a monkey. As the New York Times reported in 2000, “capuchins have learned to poke around in tree bark or termite mounds to extract a wriggling specimen of Orthoporus dorsovittatus, a millipede rich with powerful defensive chemicals called benzoquinones.” The monkeys, per the Times, rub the millipede all over themselves, transferring the benzoquinones to their fur. Mosquitoes are repelled by benzoquinones, and therefore, they leave the monkeys alone. Once again, everyone is safe — except, again, for the millipedes.

From the Archives: Hey, Let’s Crash Two Trains Together!: A really bad idea that does not involve millipedes.