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Comment by symmetricsaurus | Hacker News Reader

I am fairly certain that trains do not need the slack between cars to get moving. Like others have pointed out, trains seem to be perfectly capable of starting on inclines where there would be no slack.

There is one assumption in the article that is certainly wrong: "...but it seems crazy to think that the train’s friction coefficient is 10 times more than the cars"

This does not make any sense. The forces we are comparing is the friction between the rail and the wheels in the case of the engine and the friction in the bearings of the axles in the case of the cars. There is of course some frictional force from the deformation of the track and the wheel as well but as they are both quite rigid (being made of steel) it should not be very significant.

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The article has the physics completely wrong.

Static friction is what opposes movement between two surfaces that are static next to each other and kinetic friction is what slows down movement between two surfaces. Static friction is always >= kinetic friction.

This is explained very well in here (the top picture says it all): http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html

Think about how when you start your car from a standstill you need to be careful about your clutch release but when the car moves even the tiniest bit you can even start in the second gear if you want.

So, the engine car's wheels get kinetic friction (because they're rolling) and the other cars' wheels (which are stopped) are blocked by static friction. When you have slack and you move one car at a time you only need to beat the static friction of one car at a time. Even if the difference is 10%, if you have 10 cars and it starts to add up. I'm not saying slack is required for starting the train, just that there is an increased difficulty in doing so without slack.

Also, this is why doing a wheelspin is never a good idea when you have trouble starting your car/train. The traction when the wheels are blocked but the engine tries to move them is always a lot better than when they're spinning in place.

To be super-pedantic (because the topic is interesting), a little spin is good for an automobile. Rubber friction is a combination of adhesion, mechanical deformation, and wear.[1] In order to maximize traction with a tire you will have some optimal slip, increasing the "wear" component beyond what is seen in "normal" driving conditions. Of course, too much sliding/wheelspin and you reduce the other components. It's a balance that depends on rubber compounds, tire construction, and road conditions.

[1] http://insideracingtechnology.com/tirebkexerpt1.htm

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