How do trains pull so much weight? I’ve seem them with hundreds of freight and gas tanker cars.
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It has already been answered in a way, but I’ll say it simply:
The answer ELI5 is that it’s a metal wheel over a metal rail, meaning very low friction, something like doing ice skating. You can pull a 20 tons train with your bare hands thanks to that.
Most trains are using electric motors as their drivetrain, even really old ones. This is because it’s a lot easier prevent spinning with an electric motor than an internal combustion engine. They’re just powered by a big ass diesel generator that give off electricity..
But hey, I dunno, my sources are from Sheldon in The Big Bang Theory
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.
One thing they do is leverage momentum. When they come to a complete stop, they can’t just press the gas and go forward like a car. There is enough slack between each car that the train can slowly pull forward and starting pulling each car one at a time. If it was just one big long thing that was completely rigid, it could not function.
It’s all about producing more pulling force than friction will hold you back. A single train car doesn’t produce that much friction, so in order to maintain speed, the engine just needs to match the amount of force that all friction.
In order to speed up (ie starting the train) it doesn’t actually pull the whole train at one, rather all of the train cars are scrunched together slightly so the engine starts, and it pulls the first car, then the second, then the third and so on so it doesn’t have to overcome the initial inertia and static friction of every car at once. Once the last car has started, the engine is already up to speed.
Hills are where trains have trouble. The maximum railroad grade in the world is 13.8%. The ideal maximum grade for a massive freight train is about 1.5%. This means that for every meter the train moves forward, it increases in elevation by 13.8 cm.
If a train does need to go up a steep grade, it gains speed before hand and uses that momentum to carry it up the hill. Freight trains are also long enough that most of the time they need to go up a hill, they aren’t bringing the entire train up the hill at the same time, rather they are pulling a fraction of the cars up, where as the majority of the train is level.
The portion when the locomotive needs to develop the greatest force is when it starts from a dead stop. If the locomotive started the entire train at once it would not be able to do it, so the connections between cars are loose. This means it takes longer for the final car to start and deceases the force required. That’s why you hear that extended crashing sound when a train starts. It’s like a cushion reducing the force when you fall. It takes a smaller force to stop or start motion when the time is long. To move a train at constant speed on level ground only requires a force equal to the rolling resistance and air resistance to the trains movement. These are bothe fairly low. Then going up hills is a different story. That’s going to require a force equal to the component of the weight opposing motion. Depending on the angle of hill that could be major. That’s why they need several locomotives on big trains. To make it up hills.
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