Why Can't Trains Go Uphill? | James May's Q&A | Earth Lab

hello and firstly my heartfelt thanks to all of those who have subscribed to head squeeze it really is genuinely very important to us it's also rather flattering and now why can't trains go up hills now the smarter ones amongst you will have recognized already especially if you're a qualified railway engineer this is a bit of a trick question because of course trains can go up hills they're just not very good at it which when you think about the topography of most of the world is clearly a bit of a problem human beings can admittedly rather sweat early motivate themselves up a gradient of around 80 degrees or one in one and a quarter you do it every time you climb the stairs cars can manage gradients of up to about thirty percent or one in three before their tyres start to lose grip or their gearboxes explode into oily shards but as any 1950 school board would tell you the steepest mainline gradient in Britain is the fearsome licky incline in Worcestershire that is a gradient of just two point six five percent or one in thirty seven and a half in the old money and yet that was considered so terrifying in the days of steam that a train would often have to be pushed from behind the banks to use the technical term by up to four other locomotives the problem is one of traction or rather tractive force because for all their size and immense weight trains really struggle to find grip on a level piece of track a train is a very efficient thing the wheels are made of steel and so are the rails if you rub two pieces of Steel together you will find they produce very little friction much less than you'd get between say a tire under Road or even much less than you'd get between your finger and a piece of glass train wheels are also very narrow and profile in such a way that only at part of them rests on the track if you take a typical 12 wheeled locomotive with the weight of a hundred and twenty tons then the size of the contact area between all those wheels in the track is about the same as two 50 pence pieces which the people watching in countries outside Britain is two coins roughly so big which explains why trains are so good at slicing the hands and feets off distressed damsels who have been tied to the railway line by Edwardian ruffians it also explains why trains are able to move at all the entire weight of the Train presses the wheels down into the rails and creates grip even though it's steel on slippery steel the tractive force has to overcome the weight of the Train friction and as it starts to go faster aerodynamic drag there comes a point where the aerodynamic drag has built to a point where it's equal to the tractive force and at that point the Train has reached its maximum speed because weight is so important to the generation of tractive effort it is possible that a fatter less powerful locomotive can generate more of it than a lighter but much broader one the upshot of this is that on a nice straight level flat piece of track a train can reach a surprisingly high top speed despite a limited power output the Eurostar only generates something like 22 brake horsepower per tonne or less than a small city car with five fat blokes and a month's supply of pies on board and yet the Eurostar can do 200 miles an hour the car will struggle to crack 70 partly because of the inefficiency and the drag created by its big rubber tyres but on hills which the car will crest gracefully probably in second gear then the train will run into problems because the gradient will add to the drag created by its weight but the tractive force remains the same if it's powerful locomotive it might well start spinning its wheels if it's a puny one it will probably just grind to a halt and then the guard will come on the town I blame the stoppage on operational difficulties or something like that but the net result is the same you miss you dinner but the trains Hills cause an even bigger problem which is how to get down them because then all the braking force is concentrated in that same tiny area where the wheels meet the rails and if it's a particularly steep incline or a particularly heavy train or even if the rails are very slippery then your train can run away so you know those excuses we've all mocked over the years mud on the line leaves on the line the wrong kind of snow it's all true actually

Channel: BBC Earth Lab

Views: 3,689,588

Rating: 4.8674431 out of 5

Keywords: why can't trains go uphill, how trains work, why trains can't go uphill, james may train, james may trains, james may, head squeeze, lickey incline, bbc earth lab, science experiement, how trains switch tracks, thomas the tank engine, trains, trainspotter, bbc, earth lab, how trains are made, james may q&a

Id: KbUsKWbOqUU

Length: 5min 3sec (303 seconds)

Published: Fri Jul 19 2013

Reddit Comments

[u/iiRunner]

Besides friction, the torque of the train may not be enough to pull cars uphill. 100 cars can weigh 50-100 times more than the locomotive itself. It depends on the transmission gearing though.

Torque of a average passenger car is more than enough to climb up even a vertical wall, if the traction was there. But cars can't climb 30% (18 degrees) because the traction decreases as cosine of an angle. So if you drive downhill or uphill, your tires grip less than on a flat surface.

[u/me_and_batman]

Contact area has no effect on frictional force. Friction is dependent on coefficient of friction and normal force (derived from weight).

http://en.wikipedia.org/wiki/Friction#Dry_friction

[u/[deleted]]

May > Clarkson

[u/itsMalarky]

I'm assuming this video doesn't account for the existence of Cog/Rack railways.

[u/ImDravenUCrazy]

Serious question: Why don't they just put a layer of rubber around the steel wheels on trains? Wouldn't that make breaking and friction for hills better?

[u/QuimiQ]

Downvote me all you want but this is so wrong that I don't even want to see the rest of the video. And I don't care for any stupid excuses as to why it is OK.

[u/SinaSyndrome]

ELI5?

[u/Eric053]

I can't wait for his toy train mending video.

[u/artskin]

I can't wait for the Stig to take it around the track.