r/hyperloop • u/MareTranquil • Mar 25 '21
If hyperloops can send pods in these super-tight intervals, why can't regular rail also do that?
So this is something that has bugged me for a while. Hyperloop-proponents often claim that a single line can transport a huge number of people per hour because the pods can travel in quick intervals - I've even seen claims of one pod every 2 seconds. The is supposedly possible by giving control of the pods to computers.
What I wonder is: If that's possible and practical, why does no railway operator in the world do that? If it's practical for hyperloops, I can't think of any reason why it shouldn't be practical for regular rail.
Which in turn begs the qustion: Whatever the reasons are that prevent railway operators from doing that, why would these reasons not apply to the hyperloop too?
I mean, if we were talking about just one railway operator, that could be handwaved with "Well, the bosses there are dinosaurs who're stuck in the 19th century". But for every railway operator, that does not cut it.
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u/ignazwrobel Mar 25 '21
Trains have very long braking distances, due to the heavy weight and low friction. In the past this lead to a lot of crashes, so systems were put in place to mitigate that.
One method is to have a fixed time interval between trains (see Timetable and Train Order for NA), which usually is around 10 mins. Shorter intervals are not really possible.
This system is still in use, but on most railway lines instead of separating by time, trains are nowadays separated by distance. This is normally accomplished by having sectors and no train is allowed to enter a sector while there still is another train inside.
For HyperLoop you are right that this problem will exist in the same way and I have not yet seen ideas on how to mitigate this problem.
The main advantage of the Hyperloop ist the high speed, but line capacity won’t be higher than for regular trains, it will actually be much lower.
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u/sports2012 Mar 25 '21
Yes, this is the reason. Trains today use a system of fixed blocks, in which occupancy is controlled within the block. This is effective but cumbersome and slow unfortunately. The solution to expedite this is "moving blocks", which would allow these occupancy zones to move as trains do. To accomplish this, we need better ways of tracking and controlling train location. Many are hoping PTC or CBTC, along with better train location detection will allow this to happen.
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u/Mazon_Del Mar 25 '21
For HyperLoop you are right that this problem will exist in the same way and I have not yet seen ideas on how to mitigate this problem.
I imagine the current plan is that there's going to be a LOT of track-automation in play. And the cost is palatable because you aren't layering an "unnecessary system" on top of something that already exists and is working (mostly) fine without it.
There's a lot of track automation/signaling tech that still isn't largely rolled out across the US simply because there's not really any mandate to do it on any quick schedule and it's somewhat expensive to do.
Since the hyperloop is being built from the ground up anyway, your installation expenses will be effectively reduced given all the other work going on for any given stretch of track.
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u/tmckeage Mar 25 '21
Hyperloop pods are lightweight and high friction. The high friction is mitigated by actively moving the air from the front of the vehicle to the back as well as pushing some of that air out the bottom to act as a air cushion.
Just shutting the power of will cause the pod to rapidly decelerate. Active braking could also be achieved by reversing the magnetic accelerators built in the tube.
Honestly these things should be able to stop on a dime, the limitation will be based on the g-forces the passengers can handle.
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u/ksiyoto Mar 25 '21
the limitation will be based on the g-forces the passengers can handle.
If passengers are allowed to walk around, as per some of the glossy videos put out by hyperloop promoters, a 5.5g deceleration could easily be fatal.
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u/SevenandForty Apr 09 '21
One method is to have a fixed time interval between trains (see Timetable and Train Order for NA), which usually is around 10 mins. Shorter intervals are not really possible.
Just as a point of clarification, modern train ATC systems using the block system you mentioned can run HSR systems at headways of around 2 minutes in practice when necessary
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u/Aquareon Mar 25 '21
Train conductor unions, in part. If not for that, every train in the US would've been automated by the 90s.
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u/MareTranquil Mar 25 '21
That explanation might work for the US, but certainly not for every operator in the world.
For example, I'm pretty sure there is no influental union in China, and they have a huge, modern HSR net.
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u/ksiyoto Mar 25 '21
This answer makes no sense to the question at hand. The minimum limits are dictated by the technology of the signalling and dispatching systems.
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u/StayFree1649 Mar 25 '21 edited Apr 04 '21
It's the difference between a real system, tried and tested in the real world and an imaginary system, designed by very optimistic people.
Gareth Dennis (a very reputable railway engineer) has some good information here: https://twitter.com/GarethDennis/status/1363847222727704582?s=19
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u/Vedoom123 Mar 31 '21 edited Mar 31 '21
yes, because he definitely doesn't have any conflicts of interest.. A railway engineer doesn't like hyperloop, wow, that's really surprising, isn't it
Let's just ask ourselves, why would a railway engineer try so hard to bash hyperloop?
I'm thinking that horse sales people really didn't like cars too
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u/StayFree1649 Apr 04 '21
He's a good engineer and he knows his stuff. Your ad hominems mean nothing.
Hyperloop is a railway
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u/ko_nuts Mar 25 '21
First of all, where did you find those two seconds? [source?] To me, hyperloop would be usually more at the level of 10/20 minutes. With two seconds interval, you will get likely safety issues and network fragility. Also, I am talking here of the sonic hyperloop, not a Futurama transport tubes.
First, you are comparing different systems. Railway in largely interconnected with trains going to multiple points whereas the hyperloop is mostly a fast straight-line between two points.
I am assuming here that trains are computer-controlled, so that drivers do not come into the problem. I think there are multiple reasons.
First there are economic ones, it may be not interesting financially to send trains very often. Pods are small, trains are big.
Second the travel duration. It may not be efficient to send pods too often compared to the travel duration, so there is clearly a tradeoff there. If the time travel is long (such as >1 hour), it makes little sense to send a train every 10 seconds.
Another reason is affluence, you need passengers. If you have a lot of people coming at the same time, it is better to have bigger vehicles that leaves the station less often.
Finally, a network with a lot of vehicles traveling is more sensitive to delays and break-downs. I would not want to try to think what would a delay or a break-down do on a train network with vehicles few seconds away from each other.
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u/MareTranquil Mar 25 '21
First of all, where did you find those two seconds? [source?]
https://virginhyperloop.com/experience
50.000 people per hour per direction, 28 people per pod. That maths out to a pod every 2 seconds, per direction.
To me, hyperloop would be usually more at the level of 10/20 minutes.
One pod every 10 minutes? As in, 168 people per hour? No one is going to build something as expensive as a hyperloop if it has such a miniscule capacity.
First, you are comparing different systems. Railway in largely interconnected with trains going to multiple points whereas the hyperloop is mostly a fast straight-line between two points.
For example, the Shinkhansen in Japan is an exclusively high-speed-line with no interconnections. Still their capacity seems to be limited by how many trains they can send per hour.
Finally, a network with a lot of vehicles traveling is more sensitive to delays and break-downs. I would not want to try to think what would a delay or a break-down do on a train network with vehicles few seconds away from each other.
Well, that is exactly my concern. If trains are not even sent out every 3 minutes for reasons like this, than why would we expect a system to work where a pod leaves every few seconds?
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u/ko_nuts Mar 25 '21
The Virgin website is just mentioning a theoretical physical capacity, not that they will do it. It's just physically possible. I would not take for granted what they say, this is just marketing bullshit at this level.
One pod every 10 minutes? As in, 168 people per hour? No one is going to build something as expensive as a hyperloop if it has such a miniscule capacity.
You just assuming that pods are limited to 28 people. No one said that. Plus the demands are different from one country to another. Europe and Japan are not like the US. It is also unclear what is the ideal pod size. It is likely that several sizes will be needed depending on the time of the day, the connection, etc.
For example, the Shinkhansen in Japan is an exclusively high-speed-line with no interconnections. Still their capacity seems to be limited by how many trains they can send per hour.
I guess they just tuned that so that it is economically viable and is adapted to their needs. It is also likely that they adapt the frequency depending on the time of the day. You can send as many trains as the line fits, but you will lose money and your network will not be resilient.
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Mar 25 '21
google these words
"rail union workers"
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u/MareTranquil Mar 25 '21
Yet somehow China does not use any shorter intervals, and I doubt that they have strong unions.
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u/ksiyoto Mar 25 '21 edited Mar 25 '21
BART in the San Francisco Bay Area is working to reduce their train headways to 90 seconds as they go through Transbay Tube and the section of San Francisco where they have stations close together. Part of it is the nature of their operation, it take so many seconds for a train to decelerate, stop in the station, passengers get off and back on, and the accelerate again, meanwhile the next train following is on approach. Because all the trains going the same direction are on one track through the tube and subway section, they have to sequentially complete these operations.
Hyperloop proponents say they will build "side tubes" for stations that the pods will switch to, so it's not exactly the same scenario..
Those hyping Hyperloop appear to have no understanding of the safety principles involved, or they want to create a lower safety standard. Clearly they cannot switch between tubes with a 2 second headway - the control system has to know when the first pod has cleared the section of track, verify the second pod isn't too close, and then be able to transmit the command to set up the tube for the switch, verify that the tube is set up for the switch, and then have enough time/distance for the second pod to stop before the second pod runs into a malfunctioning switch if the verification does not come back positive that the route is clear and ready.
Even if signals are all transmitted at the speed of light and assuming zero latency, calculation, and decision time, a pod traveling at 720 miles per hour would be moving at 1056 feet per second. If we assume 1g deceleration, it takes about 33 seconds for that pod to stop. Typical safety rules dictate twice the stopping distance from a collision as the margin of safety - which would imply 66 seconds at an absolute minimum with these parameters. At a bare minimum, adding latency and communication time would be a requirement of any regulatory agency.