Why November is the most dangerous month for trains

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 [Music]

Channel: Steve Mould

Views: 398,947

Keywords: Explained, understand, rail, gel, lubricantion, chemistry

Id: ZEuFSw-CMzU

Length: 10min 33sec (633 seconds)

Published: Tue Nov 13 2018

Reddit Comments

[u/[deleted]]

Who would win:

8,000 horsepower locomotive based off the fastest train in America

Some leaves

Edits: formatting is hard