For bullet or fast trains, “aerodynamics” are very important. But we see differently shaped fronts on trains from different countries. There must be one shape which is the most efficient. Trains do not have a burden of putting out attractive styling like cars because people are not buying trains – only using them. So why is each one shaped differently? can they not decide something like japanese bullet train shape is most efficient – lets stick to that?
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The way you solve the for your drag coefficients is using computational methods, that has changed a ton even in the last 5 years. The specific shapes of these things are trade secrets, everyone is convinced they have the better one and what is “best” also depends on average operating conditions. Don’t idolize Japan and actually think about it.
>There must be one shape which is the most efficient
That sentence has two wrong assumptions. One no there is no most efficient shape for ALL situations. For example the Japanes design is that way because it works best for driving in tunnels. If you dont have a lot of tunnels another shape can be better but Japan does have a lot of tunnels. Similarly not all bullet trains are built for the same speeds. The other wrong assumption is that efficiency is the only relevant metric. There are other metrics like safety, production cost, sound and so on that are considered.
Every choice in engineering is a compromise. From materials to shapes to weight, whatever. Every choice has pros and cons. There’s never a “perfect choice.” Theres always going to be some sort of tradeoff.
I’m sure the shape of the trains is the same. There may be one shape that’s the most efficient, but a small loss in efficiency might have large improvement in other metric.
To answer questions like these, it’s sometimes helpful to use hyperbole to get at the reasons why people might do something – in other words, take some aspect of its design to a laughable extreme, and see why you might have to make changes to your design from there.
Let’s just worry about the height of our train. We’ll keep everything else a flat box.
So, for our flat box, is there a height that’s makes everything most efficient? Sure. It’s whatever height we need for our maglev tracks. Maybe a couple of inches high, maybe a foot high. Whatever that height is, it’s the minimum we need to have a functioning train, and that’s the most efficient height in terms of converting energy into movement.
It also can’t carry any people.
So, we have to move away from that most efficient height so that we can carry people. We could still keep it low profile, maybe we force all of our passengers to lay down and keep our train three feet tall. Now we’re very efficient, but our passengers hate us and our train has to be incredibly long.
Okay, so that’s a bad idea. We should at least let our passengers sit down. How much headspace do they need to sit? Let’s give them that. Maybe five feet tall? That’s our most efficient height while still letting people sit.
Except now they have to crouch to get to their seats, and there’s no space for their luggage. So we need to give them enough space to stand, and we can use the extra space above the seats to store luggage. Great, now we have our new most efficient height. How much walking room should we give them?
And so it goes. There isn’t an objective “optimal height,” because it’s driven by the context of how the train will be used. Is a double-decker train more efficient? Well, in an absolute case it will have a lot more drag on it than a shorter train. But if it can carry twice as many people, then maybe it’s more efficient on a per-passenger-mile case than a shorter train.
But then that only works if your train is always fully packed. If you run it half full, then you get the extra drag without getting the extra passenger efficiency.
And that’s just for one parameter – height. Once you factor everything else that goes into a design – shape, material, length, propulsion, use case, cost – there are multiple different parameters that can determine what is most “efficient,” and even then you have to make decisions that are sometimes arbitrary and may not be reflective of reality.
Different countries have different infrastructure, and thus different ideal shapes.
More tunnels? Straight line tracks vs curves? How wide is your rail gauge? Long distance train travel, or only/mostly short-distance with lots of stops….
Note that there are several different shapes of bullet train in Japan. Different Shinkansen train types are used on different Shinkansen lines.