Why did the train need to keep moving?

The idea that by moving it prevents people from getting off and on makes sense. But also if the train remained still, it could be covered in snow. But another reason is that in a future which lacks long-distance communications, perhaps Wilford wanted to be able to observe what was happening all around the Earth, in particular perhaps a change in climate.

While the mechanics behind perpetual motion is science fiction, the idea is pretty straightforward:

It takes energy to generate force to accelerate a mass; this means that if the train is in motion, the amount of energy it will use is minimum if any (the energy is created by the motion itself (for example, the wind generated from motion). In real life, the energy created is never enough to keep the motion going because there's friction and other sources of "loss of energy", but theoretically speaking if you can keep that energy loss to a minimum (approaching zero), you can simply accelerate a train and then never change its speed and it will run forever. Perpetual engine is based on that concept.

Now it's clear why the train can't stop. It will take WAY more energy to take the train from full stop to accelerating (up to a required speed for perpetual engine to work). In the story, presumably they either DON'T have that source of energy or there was and it's used up (thus they can't stop). In the Snowpiercer situation, they could stop if they found an external energy source to move the train again (back to the original speed) and achieve delta energy zero.

Ideally though, if designing such an engine, you would want a way to store the generated energy and keep it in a "booster" so you can use it to restart the train if ever needed. If they could skim the energy out of the running electricity (so everyone only get to use 90% of the motion generated electricity for example) and store it in a cell, they won't have this issue.

It's also important to note that a real perpetual motion engine (even the theoretic version) would not allow the train to have electricity. Because the energy generated should be JUST ENOUGH to keep the train moving, the delta energy should be zero and thus impossible to actually produce "usable energy" like electricity. Energy cannot be created nor destroyed, only changed in form, the motion energy is used to keep the train in motion so what is being transformed to electricity? The movie doesn't say (because it's impossible).

Well, for actually no real reason. A train's only reason for existing is to be moving.

A train is just a convenient way to represent a hierarchically stratified society, the closer you get to the front, the higher the class and vice versa. The only reason for a train to exist, compared to a building, is to move around. A stopped train would make no sense at all.

Because of this, the train has to keep running.

It also represents the constant struggles and fights both between the train interior (the living world) and the exterior world (the dead world) and also between the various social casts within the train itself. Even in the real world, from a passenger's perspective, trains have first and second class wagons so it's easy to setup an even more stratified version of this reality. From a train worker perspective, there's also a cast system: pilots, engineers, controllers, etc... Finally, the constant rapid movement creates a constant pressure, an uncertainty and a sense of danger that would not exist otherwise.

Let's imagine some alternatives:

• A building: It would have to be vertical to allow for the same stratification, with the upper class on the top and the lower on the bottom. But, a building doesn't have the intrinsic constant danger a train represents, it just sits there, almost nothing unexpected can happen from one second to the other. On the contrary, a train can derail or stop, the machine has to be monitored and taken care of.
• A boat: Ignoring the fact that the whole world is frozen and a boat wouldn't sense. Everyone would think of some kind of a cruising boat, which does have some stratification but not as obviously as a train. Yet, in a boat there are several ways to get from point A to B versus in a train, there is one and only corridor that runs lengthwise.
• A spaceship: This doesn't work in a post-apocalyptic scenario.

As to the "real world justifications", they are just plain BS.

• Creating water from movement? Well, as long as you can get some ice from the outside to the inside, it will melt and turn to water by itself.
• Preventing people from the rear to get to the front by hiking outside? Unlikely as it is -120°C outside (-184°F), no one on the train has the technology to survive that kind of temperature, especially those in the tail. It is also shown that simply exposing the skin to the outside air freezes it almost instantly. So no, going out of the train would be certain death anyways. Yes the movie shows 2 characters exiting the train at the end but once again, -120°C, that's simply impossible with their outfits.
• A constantly moving train is a very bad choice for surviving such a frozen and barren world. There's no one to take care of the tracks even though the cold and weather conditions can destroy easily destroy them or make them unusable. The train can and will run into storms and has no way of avoid them, it is completely at the mercy of exterior conditions it as no control over.

So, the only reason for the train to move is because of the intrinsic nature of trains, which is to move around. But, the story has to take place in a train because trains are amongst the best choices for representing danger, stratification, hierarchy without letting go of the absolute dependence on a machine and the exterior factors.

Presumably, even though it's another ice age, there would still be areas that are warmer than others. The earth is still tilted so there would be seasons. So, the train has to stay in the warmest zone of the earth to keep the train habitable. Also, there would be significant temperature between night and day. Factoring this in, the train has to circumnavigate the earth every 24 hours. The distance traveled to stay in the warmest time of day would be greatest at the equator and become shorter as you as you go further north (or south). But further the train goes goes from the equator, the more average temperature is reduced. The train would have to have a maximum speed. So, it may not be fast enough to circumnavigate at the equator. The route has to be away from the equator to be within the maximum distance that the train can travel within 24 hours.

This has become quite a calculus. For equilibrium to be maintained, all the variables have to be optimized to keep the train in the warmest area possible and maintain habitability.

This is the only scientific reason I can see for the train to keep moving as well as circumnavigate the globe.

Disclaimer: I’ve not seen the film but am enjoying the Netflix series.

Second disclaimer: I found this question by searching for “how does Snowpiercer keep moving”.

Those considered, here is a great answer to my question (which feeds into my answer here): https://www.reddit.com/r/FanTheories/comments/3dgrf3/in_snowpiercer_cw7_is_not_responsible_for_the/?utm_source=share&utm_medium=ios_app&utm_name=iossmf

Pertinent section:

The cause of the Ice Age is not some chemical, but instead the Perpetual Engine of the Snowpiercer, which for 17 years has pumped the planet's thermal energy in order to keep the train going! CW-7 is only propaganda served to the train's passengers to explain the Ice Age.

So, Why does the train need to keep moving? I surmise that the oppression of the people is maintained by the train continuing to move and maintain the ice age. This, if the train stops the people will no longer be oppressed and the opulent life of First Class will come to a violent end.

The perpetual motion engine would need to keep moving but it also keeps moving because it is perpetual. Or rather, it is until the earth’s core grows cold. Given this, one could say it needs to keep moving because it is the outlet for the earth’s core energy and so it can’t stop. Who could unplug from that amount of power without destroying everything? Wilford has harnessed the power of the earth’s core and can’t switch it off. The best the passengers can hope for is the train to become ever more inefficient and grind to a halt, allowing the Earth to warm up and end the ice age.

I would guess that given the extreme temperatures that if it stopped the moving parts would freeze up and the wheels would freeze to the tracks. It would be impossible to heat them all back up enough at the same time to get the train moving again. Much like how points can seize up in cold weather. Friction would keep the wheels moving as long as the train was going fast enough to generate the heat needed.

Maybe they don't need to be moving all the time but would need to some of the time, and therefore they have to keep the train moving or it wouldn't at all.

The train has to keep moving because Wilford wants it to keep moving, or believes it must. The analogies and technical details aside, this is pretty much the point of most of the movie, in-plot.

Wilford is obsessed with maintaining a given order. He may be benefiting from this order; and he may be correct that there is no survival without it. (Though there are plenty of hints that may not be. Nam thinks he’s not, but Nam’s also a junkie. Even if he were to say he saw signs of life or warming outside, nobody would believe him. BJH is not in the business of giving his protagonists clear moral justifications, just like his antagonist isn’t obviously callous or mad.)

All this is secondary to Wilford being a fanatic. But even if this blinds him to how good the first class passengers have it, and the suffering inflicted on the people in the tail, and even were he delusional about the train falling apart or about mankind’s chances outside the train, the train is of his design. What it does and how it does it is under his control. This extends to almost everybody living on the train - the idea of life outside of it is inconceivable to anybody but Nam.

If there was a fairer/safer/otherwise better way to do anything the train does, nobody inside would know about it, and even if they did, they wouldn’t have the means to accomplish it.

Basically, world-building in Snowpiercer isn’t really about explaining how the train works, it’s about depicting a society ruled by an absolute despot like Wilford. The train is a major instrument through which he wields power; when the movie seems to tell you things about the train, it’s mostly telling you something about Wilford. That other arrangements (like a parked train) could exist within the same world is not something it explores, because it’s about living inside a physical and ideological prison that is, in-plot, constructed around the protagonists.

The audience is meant to make its conclusions without the convenience of knowing anything those inside don’t. This is why the polar bear shows up in the very end - beyond being a cliché “they all die” ending, it’s also the first reliable piece of evidence that life is in fact possible outside the train, concluding an arc of instilling doubt in this imprisonment being justified.

Being a perpetual motion machine, stopping would halt momentum and disrupt equilibrium. The engine relies on the forward momentum of the train to keep running, and the forward momentum of the train depends upon the engine to keep moving forward. Take one away and the system fails and has to be restarted, presumably using some sort of fuel source that is either no longer available or in short supply. So it's best to never stop.

Perhaps the train needed to keep moving in order to keep up with the summer weather, which would be much shorter due to the extreme global chill. This theory follows, that if the needed to continually remain in the warmer zone, it would imply that the other hemisphere would be much too cold for even the train to handle. Having said that however, the likelyhood of this being the reason the train is constantly in motion is rather slim, considering the iregular gobal train rout map design. The rout takes it in a perimeter vector along the coasts of all major land masses. It is still possible non the less. What do you think?

Could the train (which was built for a frivolous luxury experience before the climate disaster) have been quickly repurposed when the ice age began, to circle the globe following the slightly warmer summer season?

Other than that I can only assume that if the train stopped it might be difficult to get it going again, whereupon the winter season might catch up with the train and be so much more severe that even the train could not resist that level of cold?

The director of the film; Bong Joon-ho has offered (in various interviews) his thoughts on why the train has to keep moving

It's a literal prison

• Stopping would allow people to get off

"This is true of all films, but especially Snowpiercer - the importance of space you can't emphasize enough, just the idea that these people are trapped inside this world. Of course Nam, a slight spoiler, he wants to live outside the train. He wants to escape the train. That's really essentially what this movie is about. The train is one big prison and the system that oppresses the people. That physical reality was very important and he didn't want to use CG for any of that. Of course the environment outside, a lot of that was CG. But inside he really wanted to capture the physicality of being in that space."

The setup is intended to benefit those up front

• Stopping would allow people to gain access to the front of the train.

Q. What made that metaphor -- of the poor shoved into the back of the train and the privileged riding up front -- relevant and important to you?

A. The comic book came out 30 years ago, but the concept of capitalism driving the world is still relevant today -- it’s a very universal theme. Putting it in a train is kind of like Noah’s ark, but different from a boat or plane. A train is already divided into sections, and that feeling was very key. There’s a character called Nam (Song Kang-ho) who opens gates, and it’s only with great difficulty that they can move from section to section and open up a new world each time. The trick was to differentiate from one world to the next.

I would simply say that the train needs to move to keep the air warmer. As you know, in a car, the air conditioning works only if the car is on.

If the train were to stop moving, then the movement forward could take place outside the train, i.e. people could exit in the back and walk to the front of the train, and try to revolt by breaking in (even if they could only survive outside for a few minutes). By keeping the train moving, the only way forward is the single passageway within the train, making it far simpler to maintain the social order and status quo.

Why did the train need to keep moving?

From a physical sense, the reason it needed to keep moving was that the train itself, as explained in the videos the children watch, was intended to be self-sufficient; from that, one must surmise that the train derives some unexplained benefit by being constantly in motion; perhaps this motion is necessary to somehow enable other, lesser, train functions, such as maintaining the conditions (channeling the heat from friction, perhaps?) for the interior of the train.

Note that, again from the videos, the train was not built with global freezing in mind; it was built as a luxury experience. Considering your example of the ice-breaking generating water, we can guess that if rain existed (as it does not in the film since everything is ice), the train would gather the rain in the same way it gathers ice, to replenish water supplies.

If the train didn't keep moving, whatever unexplained benefit it might be deriving from moving would be lost, and based on exterior conditions, the train might be frozen in place, rendered incapable of moving to re-derive any lost benefits. This dangerous and unpredictable scenario cannot be chanced, lest all of what is left of humanity be put at risk (from whatever environmental conditions might endanger the train; eg. an avalanche), so the train must keep moving.

If anything, it is explained that train produces water through the snow and ice picked up from the nose of the train. It would presumably need to keep moving to continue to produce water, also I believe it probably has something to do with producing power / electricity on the train

To add to John Smith's answer on the symbolic aspect of the movie, it can be noted that in the comics, the train actually can stop!

More precisely, there is a second train in the second and third tome. Larger and more comfortable than the first one, the autocratic central government keeps the power using the fear of a collision with the first train. The train regularly slows down as an emergency training to prepare the situation where the first train would be in sight. We learn at the end of the second volume that the first train was actually not a train and the government decided to harpoon the first train, in order to get a second locomotive. The second train had to stop a few hours in order to load the loco. In the third volume, the train has to leave the tracks for a short time, and to do so, it has to stop a couple of hours to prepare the train to move off-tracks.

For a review of the two first volumes of the comics. For French readers, this is a way better review.

The big deal about the 'miracle of the train' was that it was perpetual motion engine: if it were to stop, it would no longer be contributing the momentum necessary to maintain its forward velocity...

As for Why the train needed to keep moving in the first place: the entire film is a parable about society, from a dystopian perspective. In the event of a global crisis, an ultra-efficient system is created in microcosm of the society it was spawned from. With each noseward carriage the revolution passes through, the higher standard of living and thus class they experience.

The train must keep moving to keep this system in place: if it were to stop, there would be a problem with the current status-quo, and thus an examination and potential change of situation, which would not suit those currently at the front of the train/ruling class.

Sorry its such a short answer, but this is kinda covered within the film itself.