How do trains pull so much weight? I’ve seem them with hundreds of freight and gas tanker cars.
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Huge amounts of torque, usually with electric motors (even diesel trains are usually massive generators that power electric motors). Electric motors can provide full torque at 0 rpm which is very useful when trying to move something really heavy. They (edit: trains) accelerate really slowly so, while the mass is enormous, the force needed is more manageable.
The other big factor is the tracks. Train tracks are built really smooth with very gently gradients – most freight lines aim for less than 1.5% grade, meaning that the track only goes up 15mm per meter (about 1/2 an inch per yard, I think…). Because it’s so flat, the resistance from gravity is really, really small which limits the amount of force you use up not rolling down a hill when you try to start
Steel wheels in steel track have very little compression so the rolling resistance. If you compare it to rubber tires in a truck it is an 85-99% reduction.
Air resistance has the most effect on the front and rear of a vehicle. It is less along the sides of a vheicle. Even if there is a gap between wagons there is a reduction in air resistance compared to if it is separate. So more wagons/trailers are more efficient the fewer in regard to air resistance. The same is also true for trailers on a truck.
Trucks with multiple trailers are allowed in a simple location like the Australian outback [https://en.wikipedia.org/wiki/Road_train](https://en.wikipedia.org/wiki/Road_train) four trailers are allowed in some locations. That is more efficient if split into separate trucks. It is how the road network is built with limitations on for example bridges, road culvers, intersections size, and interaction with other vehicles that is that limitation.
The train also accelerates a lot slower than trucks, it would be problematic for other vehicles if they accelerated as slowly as trains. The max incline of a track is lower than roads which means less power is required to go up a hill. The speed is also in general lower for freight trains and air resistance, at least for the front increase with the square of the speed. All of this results in that trains can have less power per tonne of cargo, that is in addition to the reduced rolling resistance.
Not a train engineer, but I once read that the train car couplings have a small amount of give between each rail car. A train engine will actually back up and allow quite a few of the rail cars to back up so the train can get momentum as the couplings expand and finally catch on giving the train momentum.
Answer: the cars themselves are just big wheeled carts, not geared with drive-trains, so moving them is just a matter of generating enough force to move the mass over a distance. Locomotives use steady impulses of force from the engine and gear ratios to redistribute a massive force generated by the engine into tiny bits of work, moving all that mass of the cars behind it very tiny bits at a time until a certain speed is achieved. Because the mass is so great, it wants to stay in motion once moving through the conservation of energy, so it only takes small inputs to continually increase the speed. The fact that there are great distance between stops adds to the efficiency because there only needs to be a certain amount of pushing/pulling and then a certain amount of braking or friction to slow it back down. This is the same principle used in deep space travel impulse drives where only small impulses of thrust need to be made over time that eventually stack up to much higher velocities with minimal fuel and small engines. The orion project did somewhat the opposite by attempting to sequence nuclear blasts in order to get to orbit by using 10-15 nuclear explosions rather than the steady input of thousands of tiny pulses.
For the ELI5, it’s also worth noting that western car engines are typically larger, running at lower rpm and higher horsepower than eastern engines that are much smaller and run 2-3x rpm and smaller horsepower. For horsepower, also consider that a horse only needing to move itself can get to a gallop much faster than when it is pulling a cart, but if the cart is pulled steadily, eventually the house can run while “pulling” the cart because it’s put in work over time to get the mass in motion. These are some of Newton’s fundamental laws of energy.
Powerful engines (over 4000hp) and the fact that it takes way less energy to make something on wheels roll than it does to lift. Just think, if your car breaks down, it’s not that hard to just roll it off to the side of the road, especially once you get it going. Now picking the car up and carrying it off the road on your shoulder would be impossible because it would require so much more power. Train engines just do the same thing on a much bigger scale.
Train engines can produce enormous amounts of pulling power. The engineering of train tracks also contributes to moving these cars efficiently. An exceptionally smooth rail surface combined with hard and smooth wheel tracks and axle bearings all work together to greatly reduce the friction when they roll. Railways are designed with minimal incline or decline wherever possible, which also reduces the pulling effort necessary. Also, a small measure of inertia comes into play once the train gets moving. But it’s mostly horsepower. This is why you might notice some of the longest trains will have multiple engines nose to tail and sometimes, in the most extreme trains, even in the middle of the train. More horsepower.
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