How do trains pull so much weight? I’ve seem them with hundreds of freight and gas tanker cars.
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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.
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.
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.
Trains accelerate slowly. So even though the locomotive’s pulling pulling force is miniscule compared to the train’s mass, low rolling friction and time allow a small force to pull a train to substantial speed.
Railways are engineered to be as flat as possible.
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.
Trains are able to pull so much weight because they can balance the force applied to rotate the axels with a near equal force on the couplings.
It’s misdirection in my view to talk about torque or HP, the ability to pull load here is about traction force.
The locomotive’s engine develops a traction force, many factors go in to traction force being correct including wheel size, and the needing to safely stop.
As this is eli5 there is no math, if you wanted to do some you would need like – The wheel-rail adhesion coefficient, and to know traction force is expressed in Newtons (N) and the weight in KN.
The adhesion-limited force defines a safe upper limit of the traction force. If the traction force is greater than this limit the wheels will slip.
At very low speeds the traction force is very high – and the locomotive can pull or stop a bigger weight.
But also at low speed, especially just when movement starts, the loco needs to overcome the adhesion between the wheels and the rails, otherwise the wheels will slip, rotating in place and damaging the rails.
But why do we need such big engines –
For a railway to operate efficiently and safely, its locomotives should be powerful enough to accelerate their trains rapidly to the maximum allowed line speed, and the braking systems must be able to bring a train reliably to a standstill at a station or signal, even on an adverse gradient.
Steel wheels on steel rails roll quite easily, you can find newsreel footage of a circus strongman moving a single freight car or more from a dead stop back in the days before roller bearings. I’ve seen it done in real life by a rather large coworker, I’m not very big and I’ve done it with a chunk of 2×4 as a lever and a rock as a fulcrum a handful of times.
In the absence of rolling resistance, any force, no matter how small, can move any weight, no matter how big on a flat surface (not going up).
If you have a very small force pushing a very high weight, you get a very small acceleration. But it *will* move
Rolling resistance is a force working against movement, that comes from rolling bits being squishy (that’s why you get bad mileage low pressure tyres) or the thing you’re rolling on being squishy (I.E. driving on gravel VS road).
And trains are built with one thing in mind: reduce rolling resistance to it’s absolute minimum. So you can move immense masses with relatively little force. Steel wheels on steel tracks generate very little rolling resistance. And, if you’ve ever seen a loaded train get started, they do take forever to get up to speed. Same issue with stopping btw.
That’s also why train tracks seldom climb hard. They’ll either go around elevations, or through them with a tunnel. They don’t have the oomph to move things up. Tracks that do have significant incline will run much shorter trains, or lighter ones like passengers.
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