I traveled by train quite a lot and
every autumn I hear an announcement a
little bit like this and every time I
hear that I think I must make a video
about it because the story behind the
leaves on the line announcements is
really interesting and I'm finally gonna
make that video with sponsorship from
southeastern whenever I hear that
announcement I like to look at the
people around me because their reaction
is usually derision and you can kind of
understand why like leaves on the track
really like I can understand having
problems with snow and ice and flooding
leaves so I wanted to test it out for
myself
so I've built a kind of rig that tries
to recreate the conditions of a train
running over leaves on a rail track
after I trying to recreate what happens
on the tracks I'm going to talk about
the science of it because the science is
really interesting it's all to do with
the chemistry of lubrication so that
that's the issue right it's about
lubricating the tracks when a train
driver applies the brakes they want the
train to come to a stop in a timely
manner
so you need friction but if there's
something between the track and the
wheels that's reducing friction that's
lubricating the tracks that's going to
increase the stopping distance and it's
going to increase the likelihood of a
crash so there's the issue but is it
really an issue so my setup is I've got
a sheet metal on the ground that's
representing the tracks and I'm wearing
ice skates that's representing the train
tracks and by my calculations the
pressure of these skates on the sheet
metal is comparable to the pressure felt
by the tracks from the train wheels
above it so I'm going to take a run up
at this and see what my stopping
distance is like obviously I'm not going
as fast as a train but the idea is to
compare stopping distances under
different
so this is like a baseline like I'm
gonna tell every time I do the test I'll
try and keep my the run-up speed about
the same obviously it's not desperately
scientific this is initial testing today
I'm letting you in
and early on in the process of refining
this so here we go and take a run up one
day a movie and okay so I mean basically
I didn't travel at all a very fast
stopping distance to be clearer I'm not
exactly talking about stopping distance
because obviously my body carries on
moving forward but I'm talking about
when the blades stopped moving beneath
me and then I have to physically step
forward so that I don't fall over so the
blade landed there and then it got to
about there so okay something like five
centimeters I'll check it out properly
in a second that's basic let's try it
again now but with water so very little
movement again similar maybe about five
centimeters actual travel which is
interesting because water can be
lubricating but you know what is a small
molecule and well molecules of water
hold onto each other
that's where you get surface tension
that force can be overcome and in the
case of my skates on the sheet metal the
pressure is great enough to just push
the water out the way and so again you
have direct contact metal to metal and
there's a decent amount of friction then
the same thing happens on the rail
network which is why South Eastern well
aren't so bothered about rain so I'm
going to try it again this time with the
main corporate leaves I just got a whole
bunch of leaves from outside that are
nice and autumnal falling from the trees
I'll spread these out
okay that was quite a bit different
maybe double so run those tests a few
million times each and we're pretty
consistently getting double the stopping
distance when you end leaves which is
interesting but part me feels like
double the stopping distance isn't that
bad and maybe southeastern are making a
big deal out of nothing so I decided to
delve into the academic literature
there's quite a bit of it and it turns
out that my setup here in the studio
isn't a good match for what's happening
in the real world the important thing is
there's a two-stage process so the first
thing that happens is a train rolls over
the leaves crushing them into a thin
paste and then a component of leaves
called pectins reacts with ions in the
metal of the tracks to form a strong
bond you might have heard of paintings
before they're polymers which just means
really really long molecules and they're
used to make jam or jelly as North
American friends would call it and it's
the stuff that turns fruit juice into
the gel that you spread on your toast so
what happens next
well this thin layer of bonded pectins
and other leaf residue isn't that
slippery when it's dry but then when the
rain falls the pectins
soak up that water they absorb it and
turn into a gel it's a bit like the
lubricating strip on a razor when it's
dry it's not that slippery but when you
add water then it becomes this kind of
slimy gel so think about that you've now
got this slippery layer of gel bonded to
the train tracks and so when a train
comes along the pressure of the wheels
on the gel will squish some of the water
out but some of the water will remain
locked up inside the gel
and that's gonna create this perfect
slippery interface between the two I try
to recreate that with my setup here so I
crushed the leaves onto the metal plate
with my ice skates mimicking the wheels
on the track and then I left it
overnight in the hopes that the pectin
released from the leaves will bond to
the metal surface creating that thin
layer of bonded pectins and now I just
need to add the rain so this should be
absorbed by the layer of pectins to form
a slippery gel that's interesting
because my stopping distance was
slightly greater than just the plate on
its own or the plate with water but it
was less than when I just had the leaves
on there and look if we look up close I
think you'll be able to see why I'm
actually using mild steel so I'm getting
a lot more rust than you would get on an
actual train track I don't have
continuous coverage of that thin layer
of leaf mulch it's really patchy so I'm
just gonna take a short cut now I've
bought a positive pectin powder I'm
gonna mix that with water to create a
paste and then we're going to apply that
paste to a brand new steel sheet my
understanding is that pressure from
subsequent wheels rolling over the leaf
paste helps the pectins to bond to the
steel in the absence of that pressure
I'm going to use heat to see if that
will encourage the chemical reaction
between o2 a bit like when egg sticks to
a frying pan with a now dry layer of
pectins I just need to add rain to turn
into a gel my right foot traveled about
50 centimeters and my left foot I guess
I lifted it up halfway through which is
a strange thing to do but 50 centimeters
that that's amazing that's ten times
what I was getting on the the dry
surface and the wet surface to give this
video some semblance of scientific rigor
I measured the force required to
overcome friction with and with
that layer of gel using a fixed mass on
a dry surface it's about 70 Newtons to
overcome friction with the pectin gel
it's about 20 Newtons so I'm really
happy with that I feel like I've
genuinely managed to recreate that
bonded layer of slippery gel that you
can get on the rail network so there you
go
leaves on the line really are a big deal
this video is sponsored by southeastern
who take the problem seriously they care
hundreds of miles of trackside
vegetation throughout the year in autumn
they run these special trains with water
jets that blast away the slippery layer
of leaves they had a couple of minutes
to Train journeys in autumn to give the
driver more time to stop and start and
for some stations they run a modified
timetable if the forecast is
particularly bad more information in the
link in the description if you're still
watching this video that's amazing
you're dedicated I like that so I want
to tell you about patreon you probably
know what patreon is already but patreon
has changed in a big way recently which
is to say it now has my page on it I
launched patreon so patreon helps you to
support the creators that you enjoy in
my case if you go to my patreon page and
become a patron you can help me to
create more and better science videos
their link is in the description I hope
you enjoyed this video if you did don't
forget to hit subscribe and I'll see you
next time
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