How do trains pull so much weight? I’ve seem them with hundreds of freight and gas tanker cars.
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The miracle of the wheel and engines that pull something like the equivalent of 3000 horses (some can pull/push several times that 3000 HP). The wheel system is really the key to it though. Dragging that same mass on the ground would be close to impossible. The amount of reduction in resistance to movement (friction) provided by using a wheel is fairly complicated to figure out, but well-greased axles and the use of smooth, large wheels with small contact zones on smooth rails, is the reason that rail freight hauling (high weight load movement) is possible.
What actual friction is desired at the wheel-rail interface (where the steel hits steel) is a bit of a game, because you need a certain amount of traction to allow pulling and stopping to happen at a reasonable rate, but you want to minimize the amount of force needed to get the dang cars moving int he first place. That is, the static friction (friction when the train isn’t moving) is generally more than kinetic friction (the friction when the train is moving), so a big part of the equation is actually going to involve what force is needed to get the train moving, because once rolling, a lot less force is required. Once you have that figured out, then it is just a matter of using enough engines to provide that force. You must have noticed that long freight trains have several engines, right?
Also, although the engines have the power to get the enormous mass moving, or stopping once moving, the mass takes a very long distance to get moving fast or to stop. Also, just because there are lots of cars on a train, it doesn’t mean that the cars are all full. The number of cars a train can haul safely depends on the total weight involved, and empty cars weigh a lot less than full ones, and empty cars still need to get moved around. Longer trains likely have a lot of empty cars.
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