A galaxy wide civilization with starships. Would a single planet benefit from supersonic submarine trains to go from continent to continent. Supercavitation makes them faster than planes, but would a starship just be easier?
We need a little bit more detail on the specific circumstances. Let's take this question back one level in the world building process.
Would people from a galaxy-wide civilization settling on a planet decide to build supersonic submarines despite the fact that they have perfectly good air craft and space craft?
Probably not, because what they have is good enough to move people and things from point A to point B. There could be some wealthy citizens or government officials who are interested in submarines and decide to invest in the development of the technology. Submarine travel has a different set of challenges to design for (more on this later).
Would people who built or joined a galaxy-wide civilization decide to maintain their existing supersonic submarines despite the development of advanced aircraft and spacecraft?
Probably, people like novel and old things. The submarines could have an important place in their history and culture, a reminder of where they came from. Once a technology is perfected, it can take some time to replace it with a superior alternative. We still use foot peddled bicycles when motor bikes have been around for about a century; and we still use motorcycles when cars are arguably more comfortable and safer.
Either way, it is not specifically necessary to have the submarines. Science fiction authors like to come up with neat ideas like this, and readers like me enjoy seeing some variety and richness in the worlds that are created.
Consider these things about submarine travel:
The terrain and currents along the route need to be taken into consideration more so than when flying. A plane can take a direct route relative to the curvature of the planet, where a submarine will have to navigate around islands, reefs, underwater mountains and continents.
At least on Earth, we have abundant sea life that tends to be several orders of magnitude larger than our bird life. Hit a sea gull while flying, usually will only need to touch up some paint. Hit a whale while piloting a sub, that might leave a dent and start a fight.
If something goes wrong, it is easier to bail out of a plane with a parachute than bail out of a submarine with a life vest. Space ships and submarines are more comparable, both might have lifeboats in case of such emergencies.
A submarine traveling from say Brazil to France will technically have a shorter path than a plane flying along the same route, because that is how linear distances and circles work. Shorter radius, less distance per rotation.
Now for some ways to justify it:
If you want to say that this planet has submarines for a specific reason, design the planet so that submarines are considered the best / only way to travel to places of interest.
- Dangerous or unpredictable weather patterns make travel through the atmosphere significantly more risky.
- Environmental hazards such as radiation, toxins or particularly nasty fungal spores are on the surface, but being underground or underwater is sufficient protection.
- There are rich mineral deposits or abundant food sources underwater, and that is where it was most logical to build few cities, mining outposts or research stations.
- The skyways are so congested with traffic that submarine trains were built just to reduce the load.
- There are effectively two civilizations on the planet, one lives on the surface and reached for the stars. The other is comfortable living underground, and built submarines to cross oceans quickly. They can be at peace and have mutually beneficial trade agreements, though one will own virtually all the mineral rights.
Let's scale the problem down to our planet: we have airplane (the equivalent of your starship), yet we use cars or bikes (the equivalent of your submarine trains) to commute to work. Why are we so naïve?
- Infrastructure 1: you can park a car or a bike in a far smaller place than a 747 and you don't need massive facilities
- Infrastructure 2: as a consequence of the above, you are more flexible in where to go and also when
- Cost: below a certain distance big transports are not economically viable. Also the crew size is different, and this again impact the cost of exercise.
- Cost: on a train you can add or remove some carts to adapt to the load. On a starship you cannot remove or add volumes. And you don't want to carry around air.
I suppose the answer depends on what assumptions you're making about the civilization's technology.
Barring some radical new technology, like teleportation, it takes a great deal of energy to get something from the ground to space. So ground transportation would almost surely be more efficient than launching into orbit, circling the globe, and then dropping back down. Whatever economic system you suppose they have, energy will still cost something, because it takes natural resources, human time, etc. Perhaps the society is so rich that this cost is negligible.
For some trips, the time required to go into orbit and come back down would be more than the time it takes to travel on the surface.
Like, today we have airplanes, but people still routinely use cars. Why? Because travel by air is more expensive than travel by car. And for short trips -- like a few miles -- you can get there faster by car.
If somehow this civilization has made space travel very cheap, and there are only a few isolated outposts of civilization, I could believe that it's cheaper and easier to travel between them by spacecraft than to have trains. For example if there are only two cities, and they are on opposite sides of the globe, the cost of going to orbit and coming back down might be less than the cost of maintaining a railroad that goes around the world but only has two stops.
Depending on the surface weather this could be a very viable mode of mass transit between enclaves. A planet with high winds or a corrosive atmosphere could still need to be colonized to get access to rare elements or something. Staying deep under a body of water can minimize the impact of this atmosphere, which would be difficult for airborne vehicles to negotiate. Even a planet bathed with high levels of radiation could benefit, as the water (or whatever fluid these trains are travelling though, the supercavitation principle applies to all fluids I believe) would be protected from this as well.
A place like Europa, with oceans under ice, would definitely use this tech.
If there is not a well developed marine ecosystem these trains could move around with relative ease, but if there are established alien or transplanted Earth species, especially larger animals, then there will be significant ecological and humanitarian (such as it applies to non-human organisms) impact since these vehicles will be quite disruptive.
Would a single planet benefit from supersonic submarine trains to go from continent to continent.
When the Cost-Benefit of supersonic submarines exceeds that of starships, it'll benefit the planet to use supersonic submarines.
Specifically, when the cost of using starships to transfer goods between colonies, stations, and other points of interest on a given planet exceeds that of using supersonic submarines, the latter becomes the cost-effective solution.
Where would we see this?
1. Ocean Planets.
Despoina in Mass Effect (where the leviathans live), Atlantis, etc... Cost-benefits will favor submersibles when the civilization is not only spread across the ocean's surface, but also makes use of submerged colonies or stations, and/or mine resources at the bottom or beneath the ocean floor.
2. Subocean Planets
This is fairly self-explanatory. This includes planets with a thick surface crust that hides a subterranean ocean or planets with large quantities of subterranean seas and oceans like that seen in Star Wars Episode I's Naboo. In such cases, spaceflight and atmospheric travel is either unfeasible or may be more costly than a direct trip through the subterranean oceans.
3. Oceanic Planets with frequent storm activity.
Kamino in Star Wars Episode 2. Oceanic planets with frequent storm activity will pose a problem to starships and atmospheric flight in general. While small storms may not be an issue, particularly larger storms such as Hurricanes, Monsoons, Tropical Storms, or super-storms as we've seen in films like The Day After Tomorrow, or in Stargate Atlantis' The Storm will present logistical or physical obstacles. For civilizations such as Star Trek and Star Wars where spaceflight is cheap and powerful employing high technologies such as deflector shielding and/or antigravity, this can be limited to eliminated. For galaxy-spanning civilizations without such robust technology (such as Mass Effect) this will not be the case. In that case, development and use of supersonic submarines would be of some or great interest to inhabitants. This would be proportional to the severity of the storms experienced, be it tidal waves that disrupt landing platforms, electrical activity in the upper atmosphere (lighting releases X-rays for example), wind speeds that make air-travel difficult, or air-pressures that make flight impossible (Day After Tomorrow).
4. Planets that otherwise disrupt starship activity.
Miller's Planet in Interstellar. The gravitational tidal waves from the nearby blackhole are extremely powerful and are liable to destroy starships entering or leaving orbit that do not take care. Additionally, the time dilation effects of going in and out of the atmopshere would pose issues to inhabitants on the surface.
Oceanic planets with an atmosphere similar to Venus, or a magnetosphere similar to Jupiter would also disrupt space travel extraordinarily. Typically, it may become cost effective to limit trans-atmospheric flight in favor of ground and water transportation. If the planet is also of an oceanic type, then water and submersible becomes far more effective.
When the Cost-Benefit of starships exceeds that of supersonic submarines
Specifically, when the cost of using supersonic submarines to transfer goods between colonies, stations, and other points of interest on a given planet exceeds that of using starships, the latter becomes the cost-effective solution.
Where would we see this? Typically on worlds where starship travel is not disrupted. More importantly, this becomes much more present in galactic economies where starship travel is cheap. As we saw on Kamino (Star Wars Episode II), that is the case; but, as we see in Stargate Atlantis or Mass Effect, that is not always the case for civilizations that encompass a galaxy. A galaxy wide civilization with starships. Supercavitation makes them faster than planes, but would a starship just be easier?
Note: I'll provide more links to examples later today.
A supercavitation train would create intense shockwaves in the water and destroy, disorient or deafen the underwater wildlife in a large radius.
Think about the whales...
Environmentalists will shut it down, perhaps even eco-terrorists will bomb the thing.
Also you would have to put the train inside a tube, because hitting a whale or any other fish at these speeds means a lot of very dead passengers...
But if it's inside a tube, then the internal media can be vacuum! (like in one of Elon Musk's crazy plans).
This wouldn't work either, due to being immensely expensive...
If it's a galactic civilization, then they would find it very easy to use an orbital plane. It's just a small spaceship, you probably can buy one at the dealer down the street. No air resistance, so it would be very fast.
It's much easier to go supersonic through air than water.
Now since you said "train", I presume you are talking about some sort of fixed guideway. And here's the point that laid the passenger railroads to ruin --
Airplanes don't need a roadway.
When you have starship-level energy sources, it becomes a simple matter to have an airplane that is somewhat more ICBM than airplane, and flies to an appropriate spot where it ballistically blasts out of atmo' and then controls a re-entry in some way, possibly with engine thrust.
There's just no reason to drill through thick, sticky water. Doing so will make a LOT A LOT of heat, which could also cause problems for the ecosystem.
What's the hurry? Even with present technology, there's no real need to travel from continent to continent, except tourism. (Pretty much everything else can be done on line.) So if you're a tourist, why not take your time and enjoy the trip?
Second factor is that the time taken for actual high-speed travel is only a fraction of the total, as we could see with the Concorde. You still have to get to the airport, go through security & boarding, the plane has to taxi for takeoff... At the other end, you reverse the process, going through customs & immigration, picking up luggage, getting from the airport to your ultimate destination - and that's if everything goes perfectly.
Yes! Because they're awesome.
Nobody needs to hang-glide, or play video games, or run electricity through guitars.
Any sufficiently-advanced civilization will have jet packs, supersonic underwater trains (in tunnels, I'd guess, for the sake of the whales), and sex bots, and many other nice things.
This does not establish why they are necessary, only why they will exist. But once they exist, they will become an integral part of someone's business model. You don't think of semi trucks being indispensable, and they're probably not, but it would be an economic disaster if they all disappeared tomorrow.