The armoured trains need to be made viable without just removing other aspects of warfare. Tactical pure fusion nukes with a yield of under 75kt are common. The environment of the planet is earth like. There are a few areas that are just barren flat rock & a few very dense megalopolises. Rail infrastructure is plentiful. The primary opponents are varied but the main situation for use is between developed industrial powers. The armoured trains have to be used in direct combat & be heavily armoured. They can carry other vehicles & infantry. The standard rail gauge is 1,600mm. The technology level is near future. What could allow armoured trains to work in this setting?
If you want to stop the train you don't aim for it, you just target the railway.
No matter how thick is the armor on the train, a single bomb on the track will stop not only the one train you target, but the whole traffic on the line.
Which is a very effective way of using bombs.
If you want to prevent this, your only option is to have an underground railway, which is going to cost like a whole nuclear program if you want to cover your whole country (unless your country is a city state)
Almost impossible - rails are vulnerable
Railway lines are a very efficient means of transporting goods. However, they are a terrible option as a propulsion for equally armed primary combatants, even without nuclear weapons being involved in the mix.
- Trains can only go where there are railways and railways can only be built slowly. Therefore, it is practically impossible to achieve strategic surprise.
- With limited exceptions at junctions, trains can only go forwards or backwards on railways. This makes predicting their movement extremely easy and mining their path trivial.
- Most importantly, railways are very easily damaged. Dumb bombs can do it with good aiming, smart bombs and sabotage teams with explosives can do it trivially. (Nukes are overkill and make it difficult in the long term to utilise the area.) If combatant A cuts all the lines between combatant B's armoured trains and the area to be contested, combatant B cannot even participate in the battle.
- If nuclear weapons are used and burst to one side of a train sitting on 1600 mm gauge rail, the train will be much more easily blown over than squat, tracked vehicles sitting on the ground.
- Railway lines running through tunnels will be protected from observation and fire, but armoured trains in the tunnels cannot fight (except point blank against something in the same tunnel) and enemies can collapse the tunnel openings as an alternative to cutting the rails at the tunnel mouth.
- The weaker side just needs to build their railways with a different gauge and their enemy's armoured trains cannot enter their territory without rebuilding the rail lines.
The only situation in which this would make even limited sense would be where the railway is protected by treaty / tradition and no one is willing to attack it. However, given that the weapons that one armoured train would need to use against another would almost inevitably damage the track, the first party to start using such weapons would either win trivially against their tradition-bound opponents or be stomped flat by an alliance of all other powers.
Since modern military combat is all about air superiority, it is likely that this will stay the way in the future (upgrading to space superiority eventually). In such an environment, there is no way heavily armored ground targets work, because a single plane with a nuclear missile can simply take out the train (or the tracks). This means to realize your train-warfare future, you need to eliminate heavier-than-air flight.
Specifically, you could do this by making the atmosphere so toxic and polluted that jet turbines or light combustion engines of any type simply don't work in atmosphere because they get rapidly gunked up and destroyed by the particulate matter in the air. In one fell swoop, this eliminates fighter jets, helicopters, cruse missiles, and basically everything that lets a military project force across a long distance. Additionally, this atmospheric pollution is so bad that true stealth exists again, and it's possible to drive trains around in the smog without instantly detecting them from dozens of kilometers away.
In such an environment, warfare becomes more akin to the old-fashioned "Battleships" game, with the two sides wandering around in the fog of war, until one side spots the other, and blows them away with a railgun dart before they even know what's happening. Even better, the thick smog would heavily attenuate radio, meaning that long networks of wiring and much slower intelligence become the norm again.
It should be noted though that this smog would not eliminate heavier vehicles because the engines can be designed to handle the smog (like in a train or a large truck) and it also wouldn't do anything against vehicles that operate like a rocket and bring their own oxidizer with them instead of using the smoggy atmosphere.
A much more sophisticated ability to repair tracks.
Railway trains that can repair tracks are a known thing. and more sophisticated material science, AI, and material science could improve this ability. A train might be designed that could replace small breaks in a railway without slowing down, causing small scale disruption to matter less.
A bit more expense, in making them capable of driving on their own to some degree.
Dual purpose vehicles are already well known. Your armored trains should be designed for limited off rail use, so that if someone nukes the track they can drive off, and have the individual carts avoid spinning off. Good AI drivers and small motors should help with this.
This would help with the repair. A train could drive over a broken track, if slower, and lay down a new track along the line, ensuring that other trains could move more quickly.
Armored AI tracks.
It's easy to imagine a track system that withdraws the track under a concrete bunker if they detect an enemy bomber or are ordered to by command because of an attack.
This would lower the risk of damage from bombs. They'd still be a threat, but they would need to be big bombs and hit completely on target, or be very fast and be a surprise.
More sophisticated anti air capacity for missiles.
Missiles from trains should ideally be a serious threat for any airplanes flying a few miles above. It should be a bad idea to just fly an airplane over a train to bomb it.
With all these, they could keep being a serious threat.
Make the war over what's on the trains
They're fighting not for land or ideologies, but for things they can only easily move by rail.
Perhaps they're carrying stocks of nuclear fuel or portable reactors, with all the heavy shielding that entails. Perhaps they're moving the nukes themselves, or some other kind of advanced weapon or technology. Things that weigh several tens of tons and would be better captured than destroyed. In addition, they're moving through a neutral country or they're operating under a cold-war style situation where a large scale deployment of forces may be undesirable.
They can't destroy the track or derail the train, as it's the only way to move the thing around and it would cripple their own ability to shift it. Much better to roll up next to it in their own train and hijack the engine. Cue heavily armored attack and defence wagons to assist in and defend against these hijacks.
Justifying Warfare on the Ground
It seems your setting is capable of fusion power, judging by your statement of:
Tactical pure fusion nukes with a yield of under 75kt are common
So the first problem you need to solve is why is warfare being conducted on the ground and not in the air. Fusion power would provide the power necessary for flying battle platforms akin to the helicarriers seen in the Marvel movies. Satelites with high powered lasers would also be entirely feasible with fusion power.
Flight and orbital platforms would give any side a considerable advantage over ground based forces, so why would everyone be using trains or any other ground based force?
A few suggestions to explain this:
The atmoshpere is covered with high altitude magnetic storms, making flight difficult and target acquisition almost impossible.
Ground-to-air attacks are significantly more effective than air-to-ground attacks, either through the weaponry used (which would be difficult to justify, as kinetic bombardment is extremely potent) or through defensive systems used (eg orbital bombardment shields that are effective against overhead attacks but not so effective against lateral attacks).
The planet is covered in a shield of somekind, protecting the surface from orbital attacks (which would solve orbital platforms but not flying attack platforms or fighter jets).
Thin atmosphere, which results in a poor ability to fly conventionally.
Any combination of these factors could give you a situation where ground based combat would be preferable to air based combat. There could be other elements that make air combat unfeasible, but that is almost worth an entire question in itself.
Solving the Track Problem
Next problem you have is the tracks.
Trains require tracks to travel, and the tracks are stationary targets that would require a huge investment to protect. Even if the entire surface of the planet is covered in train tracks, enough bombing would eventually render trains useless.
So I am proposing a train that lays its own tracks as it goes along.
Something like this: https://www.youtube.com/watch?v=tMXfU8blPMM
Why two rails when one will do?
Finally, I would recommend moving over to a monorail system, as this would be easier, faster and cheaper to build and maintain than a two rail system.
With the potential for blowing up tracks and disabling your opponents that way, repairing and rebuilding tracks would be critical. If you can repair and rebuild faster than your opponents then you have a significant advantage. Placing a single rail down is easier than placing down two rails an exact distance from each other.
Plus with monorails you have the option of making them maglev compatible, allowing you to send trains down your tracks at speeds far exceeding those of trains that make physical contact with the track.
As other answers have pointed out, against an opponent with modern weapons rails are vulnerable. But this is your story, and you can find a way that is somewhat plausible. One way would be:
- Air power is very important
- In the real world air power decides interstate conflicts, so this is easy.
- Air strips are more vulnerable than rails
- Planes are really stuck if they cannot land, but trains can wait until the rails are repaired.
- Rails are harder to damage than airstrips. Perhaps hardened sleepers help. Perhaps self repairing rails. Built in IT that informs the commanders of the state of the line in real time.
- You have significant investment in rail repair, some attached to the front of the train, some in the auxiliary military vehicles.
- Perhaps they can "step over" short missing sections of track.
- Trains make great land based aircraft carriers
- If airstrips are vulnerable, make them mobile. Mobile airstrips = trains. This seems quite believable, as trains are the right shape already.
I can imagine a world where land combat is more like naval combat (as desert warfare was in WW2). Your trains are the primary assets (equivalent to naval aircraft carriers), your planes are the primary weapon, and the other vehicles & infantry are there to protect the trains and repair the rails when you need to move. Damage to rails restricts mobility but not the ability to launch aircraft.
Sure, your train can carry spare tracks to replace small sections of rail. So what happens when the enemy targets something like this:
Or uses some explosives to take down part of this ledge:
All the best automated rail laying equipment in the world with all the resupply of rails one needs is utterly useless if you don't have anything to put them on.
There is many other answers which sugest why it is hard made trains viable and how to improve that. I would try to be optimistic and propose this (which may not be totally scientic, but at least little beleivable):
- sandstorms - lot of them and lot of really tiny sand in air
- visibility is low, if the sand is also iron rich, it may disable radars (not much sure about it, sounds as good excuse)
- as sideeffect there is a lot of iron around so it may be cheap and so track and armored trains, instead of (there expensive) asphalt roads
- anything moving fast is eroded by the sand, the faster moving, the more eroding
- aircraft can lift only a little of armor (for its weight) and cannot fly fast a long way (so carry it in the train, if needed and release it just near the target)
- aicraft jet motors and/or traditional propellers have extra hard time and need even more service and fast replacement
- they say, that if you fire a gun in heavy sandstorm, the bulet is eroded to nothing before it hits ground, so explosive ammunition if better also heavy armored and used on shorter range
- rockets are also only short range weapons
- anything moving fast is eroded by the sand, the faster moving, the more eroding
- visibility is low, if the sand is also iron rich, it may disable radars (not much sure about it, sounds as good excuse) - as sideeffect there is a lot of iron around so it may be cheap and so track and armored trains, instead of (there expensive) asphalt roads
- Tactical nukes are common.
- so there is a plenty of areas, which are heavily poluted
- so you near protection not only agains sand, but also radioactivity and all kinds of toxic, that can get in air
- armor and filters and all the protection for longer traver is heavy and clumsy
- so trains are good long distance carriers, as they are really effective on rails, can carry heavy armor and weaposns and engines and filters and big load of anything you need or want (be it in war or peace for civilian purposes)
- so while other vehicles (from cars, tanks to aicrafts and missiles) are more manuerable and what all, it is way easier to move them in closed trains to the place of usage, then release then and after short range action again retrive and fix them on the way to new conflict
- so there is a plenty of areas, which are heavily poluted
- railroads are native way for communication, if the radio is often canceled by iron rich sand
- it is also easier follow the rails and stem then from sand, if neaded, than traver throught heavy terrain with heavy armor and bad navigation and limited fuel (as all-terrains in terrain have much worse economy on fuel)
- also those cities may be megacities, as this is easier protect one bid city with multilevel buildings, than many small willages
- local flora is accustomed to the weather, so it is more like potatoes, with main part underground
- if terrain allows, there are some (or even many) oasis, protected by hills and maybe even with some trees witch use relatively still air to get more sun and with deep root also more watter - which makes oasis more permanent and good for life
- so battle trains usually stops near the border and launch all other vehicles to do their work and supports them with artilery (and as rails usually leads to cities, the stabilisation of guns is easy as they usually shoots basically straight forward, enjoying cleaner air at the oasis)
- it is so much like Dune or any desert, but what if such conditions are only on part of the ground (say 10%? 20%?) - and so trains are used mainly there - and those parts are typically long and divide the land to many smaller non-desert parts - so states are formed on one or more such oasis and to conquer this place are used usually Earth-like means so it is relatively simple get whole islad, but hard to expand to another. War between states are nearly over desert, so trains are used for that. There may be much railroads between two states, as railroad takes the function of Earths roads anyway
To make trains useful targets, you need indestructible tracks.
So, let's say the planet has unusual core and mantle, with superconducting properties. Massive handwaving.
This forms magnetic field lines which allow maglev trains to travel along them, gliding above the ground, with just a tiny bit of initial lift and then ground effect to keep the trains off the ground. The crest of magnetic line is narrow and if you get away from the middle, you start getting pushed to the side, so you have to follow the exact route.
The tracks might slowly shift positions, as the configuration of the superconductor structures in the mantle shift. This could be centimeters per year in stable area, meters per day on active areas, and earthquakes could break entire routes and create new ones.
Preparing this kind of natural "track" is basically just removing obstacles, maybe making some tunnels through hills and filling up valleys a bit. Destroying the track would require making huge craters, and with heavy buldozing machinery, they might still be quickly repaired, in some cases by just filling it with water. Mining the tracks could be an option, but that's why you have the armoring and special dampeners and mine detectors in the first car. Also the trains might have limited capability to move outside the tracks. Doing that would just require massive amounts of fuel to stay off the ground. The trains might also have caterpillar-like legs, which would allow them to not only stand still, but move very slowly outside the tracks.
Anyway, the core idea idea is, the train tracks need to be non-physical so they can't be destroyed, while still being "fixed" enough so you would actually have a network of tracks. Otherwise it would just be trucks (with wheels, tracks or legs) and not really trains.
A train that could only run along tracks seems unfeasible, as others have pointed out, because tracks can be destroyed. In combination the excellent suggestions of others (e.g. trains that can repair tracks, as well as go off-track), and physical conditions to make them more worthwhile (e.g. magnetic storms, sandstorms and a thin atmosphere), might be enough to justify the use of bifunctional trains that can drive on land but are more efficient on mono/birail.
Such a train would have only the same problems as a truck or wheeled vehicle when it came to regions of track that were not easily repaired. Although tracks might often be damaged, given a huge network of tracks (even damaged ones) armored trains that could could slide on and off might be more efficient than other ground based travel options.
Adverse weather conditions above ground also lend themselves to LDutch's suggestion of a underground railway. I would add that the underground railway could have been built for another purpose before the war broke out, so it need not be a diversion of resources away from current spending on war. To prevent problems with enemy strikes causing cave ins (which trains could run into or at the best have to wait to be cleared), I would add many side tunnels that lead to the surface that a train could divert up if a cave in was detected ahead. The train would simply go over land for a short time and then dive down into the next available tunnel entrance. A decent monitoring system for cave ins of the tunnel network with lasers would allow planning for trains to go down the most undisturbed routes, and vehicles and infantry could be dropped by armored trains above ground to locations of interest.
To see the viability of armored trains, you don't have to look farther than the user of the most armored trains and one of the sole current operators of armored trains Russia. Russia once operated armored trains in the tens and hundreds during the peak of the civil war. In modern times, armored trains are primarily designed as mobile nuclear silos with missiles and officers to launch them. Their armored train doctrine is quite mature, and spans multiple technological eras. Germany operated quite a few as well, but the allies, the winners, used almost none and such most knowledge on armored train doctrine is obscure in the west. After all, trains are the most efficient form of ground transport.
There are two basic types of armored train. The standard "train" configuration with an armored engine pulling multiple cars with troops and various weaponry. Then there are Rail Cruisers which are self powered single cars with weapons, like a tank on rails. The configuration of a train and its cars can easily define its mission.
One common use of rail cruisers was attached to either end of a standard armored train. The rail cruisers would detach to scout ahead on the rails ahead of the main train to ensure its safety and rail conditions. Rail cruisers often carried a small detachment of troopers which could be deployed to operate deserted switching or resupply stations ahead of the main train. They could also be counted on the defuse or eliminate mines laid onto the tracks.
A common configuration was to attach an empty flatbed train car or two in front of a train to trigger mines and other explosives. Following that there was a combat car, either a rail cruiser or an unpowered armored train car which could fire over the flatbed. This combat car would usually be followed by the main engine(s) and followed by cargo cars and additional combat or rail cruisers attached at the rear end. this ensures protection and flexibility for the train by itself.
The flatbeds sometimes carried rail repair materials. Rail engineers can repair rails from single bomb or rocket impacts surprisingly quickly, usually within half to a full hour or so. Such repairs usually only involve two or four rail segments at most. Bridges are much more troublesome, but smaller ones could be repaired with a few days depending on damage. In modern times there exist automated rail and track laying machines. These can repair damages in minutes if the foundations are mostly intact.
As for mission, there are three main types: Fire support, transport, and direct assault. The first two were most common, with direct assaults being very rare.
The fire support mission is probably the most useful.
During the first world war armored trains could carry large artillery pieces and as much ammunition as they wanted quickly and reposition ready-to-fire faster than vehicles or field pieces. Primitive tanks and self propelled artillery couldn't match the power of railway guns in caliber or numbers.
With the advent of rockets and missiles, trains could carry a lot of punch that hit out to much farther distance. A train with a few nukes could wipe out a small nation. Resupply would just be riding the rails back to base, usually much faster and certainly more efficient compared to air or other forms of ground resupply.
These missile or nuke carrying armored trains were very elusive. They could hide in just about any rail tunnel anywhere and rotated all over the rail network in Russia. When a war started they would either roll out to launch, or wait in their covered tunnels until enemy impact and then roll out to fire off their revenge. Compared to stationary missile silos, armored trains were very hard to plug in as targets ahead of time to missiles using analog computers as they moved to often and were sometimes not viable targets even to nukes being underground and in any stretch of tunnel, which could have multiple tunnels linked.
Air defense systems in the modern era are quite advanced. An air interception system with missiles and radar mounted on vehicles can also be put on trains. While their deployment is heavily based on rail availability, trains are much faster to reposition from or to cover. The smoothness of rails can also allow systems to fire on the move with great accuracy. A modern era armored train with a wide band radar, anti-air missiles, and a radar directed point defense autocannon would be a powerful deterrent to air threats. Low flying attack helicopters overwhelming a position would just cause the train to retreat underground to pop up at a tunnel entrance somewhere else and perhaps covered by additional defense systems.
Even in WW2 automated anti-air artillery fire directors and radar trailers were somewhat common. Aircrews were trained to make speed and course corrections to fool automated flak directors used by germans. Put some on a train and they would be a large threat to bombers, who relied upon prior reconnaissance flights for target data instead of being able to ascertain most targets themselves. Back then bombers flew out to where the target was supposed to be and dropped bombs. This is why so many bombs missed their targets during that era, there were lots of navigation and recognition failures by crews. If the target moved from covered train tunnel to obscure rail sidings, trying to bomb them with overwhelming high altitude airpower would be way too expensive.
Transport is what trains are good at.
Trains are best at hauling large amounts of cargo. Armored ones would be viable if your supply lines were under attack for some reason. While probably not a good matchup versus tanks and self propelled guns, they would be fine against technicals and even air targets depending on era.
- Transporting infantry and vehicles through nominally controlled terrain is what armored trains are best for. Partisan or rebel attacks in recently captured territory would play hell on unarmored transport trucks. An armored train requires explosives to interrupt, not just small arms. This is a big step up for rebels, who usually just have some guns and maybe a few AT launchers to stay mobile.
Direct assaults are a bad idea.
Not to say it won't work in a pinch, but train tracks are pretty terrible attack paths. Even more predictable than roads, except trains can't go offroad like a tank when forced.
What can make armored trains MORE viable is the a large set of mountains and tunnels. Mountains or other terrain unsuitable for ground vehicles will make trains vital no matter the era. No matter how advanced the tank, if it can't get to an area serviced by a train tunnel, it is useless. Tunnels provide cover from air observation and attacks.
By moving quickly between tunnels or "bunkers" it would be incredibly difficult for enemy artillery to counter battery and take out the guns. Trains are fast, and surfacing between two or more open areas to fire before hiding again would be incredibly frustrating. Concealed firing ports that can be opened in the top of special tunnels would make it even more infuriating. There are many examples of artillery emplaced deep within mountains, and there were notoriously hard to destroy even with air support.
Placing cover over tracks when not in use, such as camo netting or fake trees can help conceal the exposed rail sections when not in use. A small rail cruiser sent ahead to clear rails could use a sweep or plow to clean the rails of camo ahead of the main train, then stay behind to lay it all out again once it passes.