Just passed by a train with a hundred or so cargo crates. how do they start? I understand how they continue moving, but how do they overcome the inertia of starting it? It has to be like thousands of tons, and I can’t imagine a bunch of coal being able to start moving that. unless of course it can, in which ELI5
In: Engineering
The short answer is a shitload of power/torque.
Most, if not all, engines these days are diesel-electric.
Meaning that the engine spins a generator, then the generator turns an electric motor.
1. Slowly
2. Using a lot of power.
3. On an extremely low friction surface thanks to steel-to-steel contact between wheels and rails (and good bearings).
1+3 is also why the “strongman pulling a train wagon” works.
Inertia is actually much less of a problem than friction in many cases. Speed is acceleration times time, and acceleration is force divided by mass.
If you pretend that there is no friction at all, even the tiniest force will very slowly accelerate the train. If you apply one kg-force (about the force that you can *comfortably* apply with your little finger) to a metric ton (1000 kg), it will accelerate by 0.01 m/s every second. Apply that force for a minute, and your metric ton will be moving at 0.6 m/s – about 2 km/h, or about half of a slow walking speed.
In reality there is “stiction” (a certain amount of friction force that needs to be overcome, otherwise a non-moving object doesn’t start to move), which is why you can’t *actually* get a train moving just by pushing it with your finger for a few minutes, and *some* friction (that you need to overcome constantly or the train will slow down again), but the steel-wheel-on-rails setup makes this very small compared to most other things you’re used to pushing or pulling.
If you ever pull something that’s on either ice, hovering on an air pillow, or in water, you can experience this yourself. For example, a moderate amount of force applied to a line (rope) going to a multi-ton boat over seconds to tens of seconds will make it move, and often it’s much more practical to push/pull twice as long rather than twice as hard.
Locomotives are also quite powerful – thousands of horsepowers, but unlike your car where you usually don’t use those horsepowers except for brief periods of acceleration, the locomotives do use them for much longer periods of time as they very slowly accelerate the train. They’re also optimized for applying a lot of force, and power is force *times distance* – so if the train isn’t moving much, a small amount of *power* is a lot of *force*. (This is what makes levers and gearboxes work.)
So imagine your car, times ten, in a gear much, *much* lower than first gear, absolutely flooring it, not just for a few seconds but for minutes.
The car linkages are not fixed. Each linkage has a little slack. Only one car starts at a time.
A whole lot of very smooth, very round wheels riding on top of very smooth, pretty straight, and very even ground. Round things spin very easy on smooth, straight, even track. Combine that with one or two train cars with very big engines to make just a few wheels spin.
Trains are specifically designed to pull large amounts of weight, and considering we have been doing this for a long time, the technology is quite mature. You’re right, the train is super heavy, but that’s why it’s on steel rails, to reduce the friction as much as possible to help it glide along. It’s also why trains are slow to start/stop, because unlike a car trains generally know when they’re going to be stopping, so it’s ok if it takes them a while to accelerate.
The best ELI5 answer is its due to traction motors on the locomotive. They enable the engineer to provide insane torque to the wheels at minimal rpms all while being controlled relatively easy. If the wheels slip, or spin, the engineer can apply sand at the wheels with controls from inside the cabin to give the wheels traction.
Worked for a class 1 railroad as a conductor and a carman.
The single most important reasons trains can move at all given their size is their low rolling resistance.
We rarely think about rolling resistance in our day-to-day life because as you go faster, drag from air tends to become far more dominant, while rolling resistance stays basically the same. However, rolling resistance increases with the weight of the vehicle so it becomes more relevant for larger loads.
For even the best-case scenario of rubber tires on tarmac/asphalt you need to push forward with somewhere around 20kgf (~196N) per ton and it can get way higher if the road surface isn’t perfect. That’s lost force, it does nothing but allow the wheels to continue rolling, it doesn’t go towards accelerating you, or fighting air resistance, or moving you up a hill.
For steel wheels on a steel train tack, the coefficient of rolling resistance is MUCH lower, closer to 1kgf per ton.
So for a semi-truck weighed down to US highway limits of ~36.2 tons, it requires over 700kg of pushing force just to start moving.
A large freight train on the other hand which has FAR more pulling power than a semi might weigh somewhere around 15,000 tons and require 15 tons of pulling force to get moving.
If you made a road going train using rubber tyres of asphalt, it would take 300 tons of pulling force to get moving (it wouldn’t happen)
Conversely, if you put that 36.2 tonne truck on steel wheels on a train track , you could almost certainly get it moving by hand, with only ~36kg of pushing force (this assumes perfect conditions, in practice it may be twice as hard, particularly if any wheel is riding over a joint in the railway). [Here’s a video covering this way more in depth](https://www.youtube.com/watch?v=tfA0ftgWI7U).
Combine this with the fact that locomotives **don’t** pull all of the cars from the get-go, and instead each car has a little bit of slack in the connector, allowing the locomotives to start them rolling one after the other. This spreads out the initial pulling force over a longer amount of time, thus lowering the maximum static load.
Going from 0 to 5 mph takes basically the same energy as going from 30 to 35 mph. Point being that getting everything moving from standstill is basically the same force vs inertia problem as accelerating while already moving.
(Yes, there’s a small difference with air resistance)
Moving large trains, sometimes with more than a hundred freight cars, needs enormous amounts of torque (torque is the force applied by the engines on the wheel axles to make them turn). Road vehicles like cars and trucks achieve this by using a series of gears. Automatic cars typically have 6 to 12 sets of gear combinations to achieve the desired amount of torque and efficiency. If you drive a manual transmission car you would notice that you are starting in neutral and then shift to first gear and then gradually move up the ratios as the car advances in rpms and speed. The first gear provides the most amount of torque, hence it is used to get the car moving forward. Large semi rigs carry many tonnes of load so they need huge amounts of torque. They often have 18 to 20 gear ratios. A freight train carries thousands of tonnes of freight. It will be impossible or impractical to use hundreds of mechanical gears to achieve the gigantic amounts of torque needed to get this train going. This is where electric motors come to the picture. Electric motors can generate lots of torque very easily compared to internal combustion engines. So strap some powerful electric motors to the axles of locomotives, we solve the problem of torque, along with some other advantages. All you need is electric current to run the motors. This is where the diesel engine comes in. The diesel engine, connected to an electric generator burns diesel fuel and produces the eclectic current to turn the axle motors. That is why the diesel powered locomotives are called diesel-electric locomotives. Put three or four of these powerful locomotives together, that’s all the power you need to get a 80 car freight train moving.
Hey buddy, let me make it simple. Trains start slowly, real slow. Each car in the train is started individually – kinda like a domino effect. So, the engine’s not hauling the whole train at once. And yeah, that coal’s got wicked energy – enough to get the ball rolling, so to speak!
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