Just passed by a train with a hundred or so cargo crates. how do they start? I understand how they continue moving, but how do they overcome the inertia of starting it? It has to be like thousands of tons, and I can’t imagine a bunch of coal being able to start moving that. unless of course it can, in which ELI5
In: Engineering
The single most important reasons trains can move at all given their size is their low rolling resistance.
We rarely think about rolling resistance in our day-to-day life because as you go faster, drag from air tends to become far more dominant, while rolling resistance stays basically the same. However, rolling resistance increases with the weight of the vehicle so it becomes more relevant for larger loads.
For even the best-case scenario of rubber tires on tarmac/asphalt you need to push forward with somewhere around 20kgf (~196N) per ton and it can get way higher if the road surface isn’t perfect. That’s lost force, it does nothing but allow the wheels to continue rolling, it doesn’t go towards accelerating you, or fighting air resistance, or moving you up a hill.
For steel wheels on a steel train tack, the coefficient of rolling resistance is MUCH lower, closer to 1kgf per ton.
So for a semi-truck weighed down to US highway limits of ~36.2 tons, it requires over 700kg of pushing force just to start moving.
A large freight train on the other hand which has FAR more pulling power than a semi might weigh somewhere around 15,000 tons and require 15 tons of pulling force to get moving.
If you made a road going train using rubber tyres of asphalt, it would take 300 tons of pulling force to get moving (it wouldn’t happen)
Conversely, if you put that 36.2 tonne truck on steel wheels on a train track , you could almost certainly get it moving by hand, with only ~36kg of pushing force (this assumes perfect conditions, in practice it may be twice as hard, particularly if any wheel is riding over a joint in the railway). [Here’s a video covering this way more in depth](https://www.youtube.com/watch?v=tfA0ftgWI7U).
Combine this with the fact that locomotives **don’t** pull all of the cars from the get-go, and instead each car has a little bit of slack in the connector, allowing the locomotives to start them rolling one after the other. This spreads out the initial pulling force over a longer amount of time, thus lowering the maximum static load.
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