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I have seen a video on youtube where Feynman explains that engines of trains are heavy to compensate for the fact that steel has a low coefficient of friction.

But, whereas this seems plausible for the old steam engines, new trains are significantly lighter.

How these trains are going up an acclivity?

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    $\begingroup$ The same principle operates with modern trains - perhaps they're just heavier than you think? Related/possible duplicate: Grip of the train wheels $\endgroup$
    – lemon
    Jul 8 '18 at 12:04
  • $\begingroup$ @lemon I think that they are lighter $\endgroup$
    – veronika
    Jul 8 '18 at 12:52
  • $\begingroup$ Having a heavy main train locomotive to pull the rest of the train cars probably isn't as much of an issue nowadays for the simple reason that there often isn't a main locomotive responsible for doing all the work of pulling the rest of the train. For trains like SF Bay Area's BART or many Japanese commuter trains or the Shinkansen, all of the cars in the train have electric motors and so all of the cars contribute equally to moving the train forward. $\endgroup$
    – user93237
    Jul 8 '18 at 17:41
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    $\begingroup$ "I think that they are lighter", without any evidence to back it up, is a bit thin on the justification, isn't it? $\endgroup$ Jul 9 '18 at 17:16
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    $\begingroup$ @Quantumspaghettification Even if there were a difference, it would fall to OP to do the research and show that the effect was actually real. $\endgroup$ Jul 9 '18 at 20:25
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The difference is that older trains were pulled by one engine, which provided the pulling force for the whole train. The amount of traction this engine could provide was limited by the weight of the engine. Modern trains have motors located in each carriage, so the traction force is spread throughout the train instead of concentrated at the engine.

Electric trains are lighter because they do not need to carry their own fuel. Power is generated externally and supplied to the train through electrified rails or cables.

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