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
Yes, they’re very heavy. So you have a LOT of inertia to overcome.
But, steel wheels on steel track is very low friction and low rolling resistance.
But if you have low friction, how do you get traction? Traction is a function of both weight and coefficient of friction. So train engines are just super damn heavy. And for the coefficient of friction, sand is often spraying onto the track right under the wheels.
Put all of that together, and do it with 2-3 engines working in concert, and you’re, well, rolling.
They gotta compress the whole train so all the cars are bumper to bumper. Then the locomotive(s) can start pulling, always just taking one car from standing to moving, which pulls the next car. The resistance is a lot less on already moving cars so when the entire train is moving, they can start accelerating properly. [On very long trains this makes for a giant “shockwave” travelling down the train when they start or brake](https://www.youtube.com/watch?v=7Anj8A6zgL4)
Edit: apparently it’s an outmoded practice for underpowered trains and modern locomotives have enough power to just start the entire mile long thing all at once.
There is a lot of talk about inertia and friction on here, but it’s also important to note just how powerful modern diesel electric locomotives are…a quick google suggests they have over 4000hp and over 60,000 lb-ft of torque. When you have four of those running together we are talking massive amounts of power…like multiple top fuel dragster amounts of power that can run for hours on end power…incomprehensible this side of ocean-going ships power. Power!
There’s a pretty simple equation that’s helpful here, F = ma, or in words: force = mass(for our purpose here it’s pretty much just how heavy our object is) * acceleration. As someone else mentioned, the friction of the wheels is pretty much negligible, so we can ignore it really. Trains are pretty strong so our pulling force is pretty strong, but we’ve got a TON (or more accurately, tons, haha) of mass. This means that the acceleration has to be really low. Acceleration is basically how fast an object is speeding up, so a low acceleration means that train is barely speeding up, and that’s exactly what happens, trains take ages to get up to full speed!
Inertia is an interesting topic, but it doesn’t really effect our F = ma equation up above. Basically inertia is directly correlated to mass, so an intuitive way to understand this might be to look at something on a slightly smaller scale, like a human pushing a car that is in neutral. Assuming we’re on relatively flat ground, a human can probably push something with roughly 100lbs of force or so. Meanwhile, a car can weight a couple thousand pounds, roughly 10-20x more, but a human can still push it get it moving. It’s slightly oversimplified, but it might help you understand whats going on in the train situation a little more intuitively.
The slack thing is a bit of a misconception, because stretching and bunch slack is an important factor in train handling, and if you have to slam the throttle and stretch each coupling to get the train moving you have way too little power and are probably going to get fired lmao.
To put it simply, modern road locomotives in North America weigh 400k lbs and have about 4000 (sometimes 4400) horsepower, so 3 of them running is 12k horsepower and weigh 1.2m pounds, so they’re really heavy and have a lot of grip.
(Side note tonnage ratios are different depending on territory, so 12k horsepower can sometimes take 12k tons of freight, sometimes the equivalent power can only take 8k)
Simple. Try pushing a heavy box. Then push said heavy box when it’s on some kind of cart. It’s much, much easier.
Friction is what makes you unable to push something heavy. Wheels make friction much less.
It might be slow to start and stop, but actually getting it moving takes far less than you’d think.
Here’s my five year old answer that might put a good image in your head: take a string and tie a bunch of objects to it roughly every six inches for, let’s say, ten feet. I want this to be long enough for you to really see. Scrunch all the objects close together in a straight line. Now, hold the front of the string and slowly start pulling. Notice what happens. The first object starts to move, then, once the line between it and the second one is taut, only then will the second one start to move. The first object is moving at your pace now, so it’s not resisting very much, it’s basically joined your system, so most of your energy is going toward getting the second object moving. The other objects, again, may as well not exist until the string is taut again.
Trains…. trains is kinda like that.
I have no idea but an educated guess is that the cargo trains also have acceleration?
Something nobody has mentioned yet is that electric trains use ‘traction motors’ which have very high torque at low speeds, reducing as the speed increases. So it’s not like your internal combustion engine with a clutch and gearbox where the engine needs to have a certain number of revs before it can do any useful work without stalling. And most (I believe) so called diesel locomotives are actually diesel-electric. Where the diesel drives electric motors.
When you think about the work these things can do, these are some pretty serious motors and generators.
It only pulls one car at a time. The cars are actually closer together on a stopped train than on a moving one. When the train starts, it pulls the first coupling taut, and then it starts to pull the first car. Then the next coupling gets pulled taut, pulling the next car, and so on. This way, it doesn’t have to overcome the static friction of all the traincars at once. Once the car is moving, it takes far less force to keep it moving or accelerate it.
The train actually can’t start if the cars are already taut. If this happens, the brakes at the back of the train are applied, and the train has to back up slightly. Then, once the train is compressed, it can start as normal.
These two states are called buff and draft. When they are held taut, it’s draft. When they are loose, it’s buff.
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