Assume you can hook every locomotive and car Union Pacific owns up into a single gigantic freight train. Safety and laws aside, would it work?
In: Physics
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You could get pretty far with distributed power. Just throw a few engines in every couple dozen rail cars. The limitations of how far apart those cars can be depends a lot more on the size of the yard and the build area than it does the physical limits of the train. Once you get to the length where you need to account for the time it takes for signals to reach the other engines from the control point, you’re well beyond the circumference of the Earth, let alone the longest continuous piece of track
You’ll have to space the locomotives as equally as possible to distribute the pulling and braking loads or else the stress on the first non-powered car will be tremendous, perhaps even destructive. The mega train would operate somewhat like a series of shorter trains loosely connected together yet operating in sync. The total mass would still make getting it up to speed and much less braking a real tough challenge.
Then you’ll have to deal with level crossings and track switches as the mega train will be occupying it for very long amounts of time that you’ll have to have alternative routes. Then the next limitation is how long your rail sidings are – kinda pointless to have a mega-train stretching far beyond what a load/unloading facility can accommodate. Even if you separate individual cars to be un/loaded and serviced, that would still involve stopping the entire train ever so often which ruins the timings and the profitability of the concept.
TLDR: A mega train in itself is not an insurmountable engineering challenge per se, but the limitations of the other cogs in the rail system and the economics of the whole thing would likely preclude it from happening.
Trains have a decently low level of friction with steel wheels rolling on steel tracks, but that is not the same as no friction and it adds up. There is also the friction in the moving parts of the cars themselves. Eventually the resistance generated be more than an engine can pull.
So you start adding on engines. More engines = more power to pull. But the resistance is still there and now you end up straining the connections between the cars and the cars themselves. Eventually one or the other will give. Add any sort of uphill climb over distance and you increase the weight strain.
Now, you can fix that problem by placing the engines periodically along the length of the train. These provide a type of strain relief by applying the power along the length of the train instead of only at one end while pulling the full weight like an anchor.
At this point you are limited by the length of the track. You can only fit so many on the track.
There are trains that add extra engines along the train length for extra power. There are diminishing returns because you’re still stressing all the wheels more as you add more and more train cars.
You’re more likely to run into limits of the specific lines it’s running on. Most train lines have a limit for length of trains based on the capabilities of the tracks themselves. Things like requiring trains to be able to fit into sidings, or how long a train will take to clear a crossing.
There is a physical limit to each engine and the amount of work it can do.
Yes there would be an upper limit if you mean engines up front.
No there wouldn’t be if you can place engines wherever you want. It would effectively be like long trains tailgating each other if you do that. So no limit then.
Depends on how you define the length of a single train, as others have pointed out. If you do not care about adding locomotives at several points in the chain, you’d be good to go. If you want all the locomotives at the front of the chain, you’re going to run into issues.
Adding locomotives at the front will, for example, increase the force that they can pull, which lets you add more cars. However, at some point the weight that is being pulled will be too much for the couplings keeping the train together, and the chain will break.
One would assume that braking gets more difficult by every bit of weight added due inertia, to a point where braking is so difficult, that train couldn’t take corners anymore. And brakes would likely wear and tear like mad. Rail infrastructure, too.
Also, your superlong train would take super long to load. And we don’t need or want to load trains, we need goods and passengers **delivered**. Loading and unloading is just something inconvenient. If all train mostly does is loading, it’s pretty useless. Whatever you win with longer train, you probably lose triple due delayed delivery.
Assuming we can disregard things like safety procedures, conditions of the track and rolling stock, fuel for the locomotives, assembling this monstrosity, etc…
There are two primary things that limit the length of a train. Those are:
1. The amount of tractive effort the locomotives can supply
2. The strength of couplers between cars
Trains aren’t frictionless. They go through a lot of effort to reduce friction in every place they can, but you can never escape it. So the weight of the train does add up. At some point, the weight of the train becomes so great that the locomotives pulling it cannot put enough power down into the track that it can be pulled.
Do note that the amount of power an engine can make is not necessarily equal to the amount of tractive force it can supply. The fact that railroad designers have gone far out of their way to make friction as low as possible on rails does work against the engines themselves. The friction between the wheels of the locomotives and the rails is usually the limiting factor here; if a loco pulls harder than the friction allows, the wheels just slip and the whole train goes nowhere.
So to pull an arbitrary-length train, you need some proportion of locomotives pulling.
The other thing that matters is where the locomotives actually *are*. If you put them all in the front, and trail all your cars behind, you could snap the train apart. Think about it: if you had a near-infinite amount of train cars to pull, and a near-infinite amount of locomotives to pull them, all of that force needs to transfer through the first coupler between all the locos and all the cars. That would need to be a near-infinite strength coupler. That obviously doesn’t exist
To eliminate this, you need to pepper your locos through your train, interspersing them at regular intervals to ensure that no coupler gets overstressed. Real trains already do this, and the technique is often called “distributed power”. It’s why if you wait at a level crossing for a freight train to pass, you might find locomotives at the rear end or in the middle of the train. That’s what they’re doing.
With a high enough ratio of locos to cars, and with distributed power throughout the train, there’s basically no limit to how long a train can be. If you clipped the train apart at every locomotive, you’d have a bunch of short trains that can all run on their own, With this arrangement, you’ve essentially created a bunch of short trains that all happen to be linked together.
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