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Sometimes, in this world, a small thing beats\h
a big thing. David beats Goliath, a pea beats a\h\h

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princess, a few leaves delay hundreds of trains by\h
thousands of hours a year. Yeah, as it turns out,\h\h

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fallen leaves are a huge problem for choo choos,\h
causing misparks, damage, delays, and at least\h\h

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one really annoying sound. But how? I mean, can’t\h
dad just clear the tracks once he’s done with the\h\h

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lawn? Apparently, no. The problem’s way stickier\h
than that, and all the solutions are weirdly\h\h

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sci-fi—but I’m getting ahead of myself. How\h
does this thing do such a number on this thing?\h

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Well, here’s the thing about this thing: it\h
only works if there’s the exact right amount of\h\h

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friction between these things and these things.\h
Obviously, if you want your train to zoom fast,\h\h

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you want to minimize friction. Hard metal wheels\h
help because, unlike air-filled tires, steel\h\h

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wheels barely deform under the weight of what\h
they’re carrying, meaning a tiny contact point,\h\h

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less rolling resistance, and faster rolling. The\h
coefficient of friction between train wheels and\h\h

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tracks is around or under 0.5, versus about\h
0.9 between rubber car tires and a dry road.\h\h

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But trains, rather famously, have to stop—so\h
they need some friction between the wheels\h\h

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and the tracks, to be more specific,\h
a coefficient of friction over 0.17.\h

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But when Mr. Leafy and his buddies come to\h
town, the friction on the tracks can get as\h\h

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low as 0.015, because leaves are, by accident of\h
evolution or trick of God, perfectly engineered\h\h

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to make train tracks extra super slippy.
By the time a leaf falls off a tree,\h\h

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it’s mostly made of water, cellulose, pectin, and\h
fatty acid. And when you smush the full weight of\h\h

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a train on that via contact points the size of\h
an HAI writer’s thumb, you apply a gigapascal\h\h

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of force—and no, I don’t know what a gigapascal\h
is either, but I assume it’s one billion Pedro\h\h

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Pascals punching as hard as they can… which\h
would do unspeakable damage to a person,\h\h

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and does something similar to leaves. Namely,\h
turning them into a slimy black sludge that’s\h\h

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conveniently bonded to the top of the rails.\h
Which is a problem, because trains can’t always\h\h

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brake fast enough on slippery rails, so they might\h
mispark in a station and have to reverse back in,\h\h

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or just skip it entirely if they can’t\h
slow down in time to stop there… which,\h\h

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along with messing up train schedules, induces\h
an as-yet-unstudied-but-presuambly-lethal\h\h

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amount of rage in people who miss their\h
train while standing on the platform.\h

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Along with the human psyche, leaf sludge damages\h
wheels. Sometimes, a slip-slide will trigger a\h\h

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train’s automatic emergency brake, which will\h
cause the wheels to skid along the track and\h\h

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wear parts of them down flat. And something\h
cool about flat-bottomed steel wheels is that,\h\h

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along with not working so good, they make a super\h
loud clangy sound every time they hit the rail. So\h\h

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oftentimes, they need to be replaced. All of this\h
causes delays: from correcting misparks to taking\h\h

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trains out of service for repairs. And beyond\h
that, there’s also signal problems: the sludge\h\h

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messes with signals, so trains pass them without\h
signaling to trains behind them that they’ve\h\h

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passed, so those trains get held up waiting for\h
clearance that they should already have. In 2015,\h\h

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leaf sludge caused 5,800 hours of delays on\h
rail lines in the UK. So this is a big problem,\h\h

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and the easy solutions don’t work. You can’t\h
send dad in with a rake because the leaves are\h\h

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sludge. You can’t pull up all the trees\h
near tracks because A, in the UK alone,\h\h

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that’s more than 13 million trees, and B, without\h
trees’ root systems holding land together under a\h\h

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lot of tracks, there would be landslides,\h
which would probably cause more delays.\h

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So how do you deal with this? Sometimes,\h
trains get equipped to lay a traction gel\h\h

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with sand particles in it on the tracks to\h
give themselves some extra grip. Sometimes,\h\h

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rail operators run special trains to pressure-wash\h
the lines, spraying 48 gallons of water per minute\h\h

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through a wee pinhole on the bottom of the train.\h
The UK’s Network Rail ran 61 jet-cleaning trains\h\h

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in 2021, and these were also the main system in\h
place for cleaning commuter trains in the New\h\h

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York Metro region for years, with two such\h
trains cleaning the entirety of the Harlem\h\h

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and New Haven lines every single day for six\h
weeks in the fall. This method is effective,\h\h

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but it’s inconvenient. And beyond that, it’s\h
kinda boring. Believe me, I wouldn’t be making\h\h

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this video if the coolest solution out there\h
was “rinsing off the goo.” No, dear viewer.\h\h

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The solutions here are cooler. Solutions like:
Ultrasonic bubbles! Thermal ablation with megahot\h\h

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plasma! Electromagnetic braking! Shooting tiny\h
dry ice pellets at the track that flash-freeze\h\h

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the sludge, then sublimate into a gas that blasts\h
the frozen sludge off the track! All of those are\h\h

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cool. But the coolest? That would be LASER TRAINS!
Yeah, baby! That’s right! Laser trains! Last fall,\h\h

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the MTA ran a pilot program where they strapped\h
lasers to the bottom of a commuter train. Those\h\h

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lasers fired infrared beams at the tracks wherever\h
there was leaf sludge at just the right wavelength\h\h

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to reflect off the tracks without damaging them…\h
while still completely vaporizing the sludge. It\h\h

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was a huge success: during the pilot program,\h
there was a 40% reduction in emergency stops\h\h

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from slip-slides, and wheel repair costs reached\h
record lows. This fall, they’re adding a second\h\h

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laser train to the team, meaning all vital parts\h
of the Metro North system will be laser-cleaned\h\h

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once a day, which is conveniently how long the\h
rails stay clean before more sludge forms. So\h\h

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that’s one point to laser trains, zero points to\h
leaves. It just goes to show: sometimes the little\h\h

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guy beats the big guy. But if you give the big\h
guy lasers, he can almost always pull it back.\h

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The world always has problems, some of which\h
potentially even more pressing than slippery\h\h

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train tracks, but many are getting progressively\h
solved thanks to the ingenuity of those who have\h\h

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dedicated their careers to doing good. It\h
is actually, genuinely possible to have a\h\h

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career that is both fulfilling and financially\h
sustainable—it just takes a little more work to\h\h

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find it. That’s why our sponsor, 80,000 hours, is\h
offering to help. They’re an independent nonprofit\h\h

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funded by donors and philanthropic organizations\h
that aims to solve the world’s biggest problems\h\h

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by getting people like you in the jobs that\h
will do that. The thinking is that you spend\h\h

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80,000 hours of your life working, so you might as\h
well make those 80,000 hours personally fulfilling\h\h

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and impactful. 80,000 hours provides a host\h
of resources entirely for free—a job board,\h\h

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get started on planning a career that works on one\h
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