
Hyperloop Alpha [pdf] - soupboy
http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf
======
blackjack48
Location is a primary reason why Musk can claim Hyperloop will be cheaper than
high speed rail. As the paper correctly points out, right-of-way acquisition
is very expensive. Bypassing major population centers is an easy way out of
using eminent domain or expensive tunneling to build the system. The HSR
project is required by Proposition 1A to pass through several population
centers in the state and have a couple dozen stops. Notably, Hyperloop
proposes to bypass all of the SF peninsula, San Jose, and Central Valley
cities like Fresno and Bakersfield. HSR was politically viable because these
regions were included in the project. Furthermore, the route and station map
appears to hint that Hyperloop's stations are positioned away from city
centers. For example, the "San Francisco" station appears to be located in the
East Bay and the "Los Angeles" station is closer to Burbank. This is quite
significant from an urban planning perspective as it means that short haul
transit or car trips have to be accommodated for a majority of those people
who want to use the system. It also adds to the total travel time needed to
get from door to door for a greater number of people than does HSR.

I'm not saying that Hyperloop is an inherently flawed project or that HSR is
the perfect solution to the state's long-haul transportation needs. I just
think that Musk is oversimplifying the situation.

~~~
enraged_camel
>>I just think that Musk is oversimplifying the situation.

Yes. After all, the project whitepaper has the word "alpha" in the title, and
the authors specifically say they are open to constructive feedback. Have you
thought about emailing them about your concerns? I think they are valid.

~~~
blackjack48
I'm fairly certain they've realized this. My guess is that "Proposed SF-LA
tube costs billions less than HSR" makes for a better headline.

------
alex-g
It's interesting that the biggest line item in the costing is not the tube,
nor the capsules, nor the vacuum pumps, nor the solar panels, nor the land,
but the concrete pylons that support the tube - about two-fifths of the total.
The pylons are planned to be 30m apart, making for about 25000 of them.

~~~
toomuchtodo
But that's the beauty of it! You can make those pylons anywhere, truck them
in, and install them in whatever order you want. It's construction....that
scales up almost effortlessly.

~~~
RyJones
perhaps you can pour sections and truck those in, but there's no way in hell
you're moving a precast that size on anything that doesn't float.

For scale, you know those 4' by 4' by 8' blocks of cement you see all over the
place, used for erosion control and the like? They're called ecology blocks,
they're about $20 each FOB the cement plant. You want them shipped? A lowboy
can carry two or four at a time without exceeding load limits. When I looked
into buying a couple hundred, having them shipped about sixteen miles, it was
going to cost hundreds per block to truck it out plus renting a crane to
unload/place them.

Those are tiny blocks with almost no rebar, no engineering, no preload,
nothing.

~~~
toomuchtodo
Pretty sure you can truck pylons if you can truck in precast bridge decks:

[https://www.google.com/search?q=precast+bridge+deck](https://www.google.com/search?q=precast+bridge+deck)

~~~
RyJones
Concrete is about two tons per yard. The legal limit on most trailers is about
20 tons (40,000 pounds, the trailer is about 40,000 for the legal limit of
80,000 pounds). So you can get about 10 yards of concrete in one go; you can
use specialty trailers to get higher numbers, but even if you double it to 40
tons, that's still only 20 yards.

a 4' x 4' x 8' ecology block is about five yards. So let's picture the pylon
having a 4' x 4' base that sits on granite, requiring no foundation. The pylon
is going to max out at 16 or 32 feet tall if we're required to stay within the
load limits.

furthermore, when you see those ribs moving around, they're moving around on
interstates or state roads of solid construction. If there was an interstate
running directly from San Francisco to Los Angeles along this route, we
wouldn't be having the discussion. Building such a road to deliver precast
sections is quite the endeavor.

I stand by my original claim that these would need to be cast-in-place.

~~~
jessaustin
_If there was an interstate running directly from San Francisco to Los Angeles
along this route..._

Umm, that's kind of the point. It's called I-5.

------
orofino
The focus from people on bathrooms is puzzling to me. I've been on buses,
without facilities, that go for hours without stopping. This is a 35 minute
ride with restricted movement, if you greatly fear being able to hold it for
that long, there are other, less efficient methods of transport to accommodate
you.

~~~
kailuowang
On a bus you have the chance to get off every 5-10 minutes, that's a big
difference. There will be thousands of passengers on that day. Just imagine if
you have to do this trip 10 times a day every day for 100 days, will you still
take risk?

Update (since I've been downvoted):

All these buses analogy isn't exactly the same case because they still have
the option to stop somewhere, which give the passengers the assurance that if
they really can't hold it's not the end of the world. Taking that into
consideration, I think it could solve most people's bathroom concern by adding
bathroom stops to the loop, in the case of semi-emergency, ideally a pod
should be able to make a stop upon request within 5-10 minutes without
disrupting other pods. Some financial incentive might be needed for
encouraging people to hold longer but I do see legal risks in that approach.

~~~
orofino
You're thinking city buses. I'm thinking long-haul busses (like greyhound in
the US). Greyhound buses likely have facilities, what I'm actually talking
about are the buses used a core parts of the transportation infrastructure all
over South America. Many of these also have facilities but I've been on many
where the bathroom isn't functioning or just plain isn't there. You can easily
travel for 30 minutes to an hour without stopping.

We're largely (70%) talking about commuters here, they're taking this twice a
day, most days out of the year. Again, I'm not sure why for 90%+ of people
that this would be an issue.

~~~
kailuowang
All these buses analogy isn't exactly the same because they still have the
option to stop somewhere, which give the passengers the assurance that if they
can't hold it's not the end of the world. Taking that into consideration, I
think it could solve most people's bathroom concern by adding bathroom stops
to the loop, in the case of semi-emergency, ideally a pod should be able to
make a stop upon request within 5-10 minutes without disrupting other pods.
Some financial incentive might be needed for encouraging people to hold longer
but I do see legal risks in that approach.

~~~
orofino
Financial incentive? You're traveling at 700mph through a partially evacuated,
elevated tube in order to get to work. That isn't incentive enough?

------
resu
If the U.S. doesn't take this on, I hope another country with a big appetite
for enormous public transit projects takes this on. Japan, Korea, and China,
I'm looking at you...

~~~
ianb
China seems set on a different path, but I guess you never know. Korea seems
too small to benefit. And Japan perhaps too mountainous?

Looking at California: [http://bit.ly/1eFevxA](http://bit.ly/1eFevxA) And
Japan: [http://bit.ly/19ZXwbJ](http://bit.ly/19ZXwbJ) I don't see any roads in
Japan that are nearly as straight as I-5.

The places where this would make sense have destinations fairly far apart, but
not too far apart, with somewhat amenable landscape between them. The Midwest
would work, Australia, Russia. Probably Brazil, Argentina? Probably a bunch of
connections in Africa or maybe the Mideast... but that wouldn't work for other
reasons. East coast seems a bit cramped.

~~~
thret
In Australia this could potentially be viable between Melbourne and Sydney,
but probably only if it doubles as a cargo train. I doubt there is enough
daily people traffic to warrant it otherwise?

It is around 900km though, almost a straight line along the Hume highway and
the busiest city-pair in Australia by a long way.

~~~
ddeck
_> I doubt there is enough daily people traffic to warrant it otherwise?_

Sydney<->Melbourne is the third busiest air route in the world with 8 million
passengers in 2012[1]. I'm sure there's more than enough "potential"
passengers.

[1]
[http://en.wikipedia.org/wiki/World's_busiest_passenger_air_r...](http://en.wikipedia.org/wiki/World's_busiest_passenger_air_routes)

------
sylvinus
IMHO Musk (or anyone who follows up) should drop the "passenger" version and
go with the "passenger plus vehicle" version only.

Because right now what people and the press are going to focus on is how small
the cabins are, why there are no toilets, etc. On the other hand if you can
imagine putting a car in there the claustrophobia argument disappears and you
can think straight again...

~~~
prawn
Disagree completely. I'd ditch the vehicle option. If the earliest forecast is
for this to be built in seven years, how much will the driving landscape
change in that time? Robo-cars even? This is primarily for commuters who can
walk/taxi/rent at the destination. Even travellers will do the same.

If you want to take your car, drive it IMO. When you fly, there is no
expectation of taking your car along with you and this should be similar -
focus on what we will need rather than on what a minority could fancy.

~~~
sylvinus
I agree to your point about taking your actual car. Loading cars inside would
indeed complicate the system a lot.

What I tried to say was more about the psychological impact of imagining a car
fit in the capsule, even if the same space is just taken by larger seats.

------
cocoflunchy
Interesting quote:

    
    
        The intent of this document has been to create a new open source form of 
        transportation that could revolutionize travel. The authors welcome feedback 
        and will incorporate it into future revisions of the Hyperloop project, 
        following other open source models such as Linux.

~~~
landhar
I would really like to see people starting to build small scale models from
this design. It could become the new version of building a rocket in your
backyard.

------
Mindless2112
Strange PDF title -- "Falcon 9v1.1 - Reentry Nozzle Study" \-- is there some
design reuse or just document reuse?

~~~
waskosky
A funny mistake most likely, probably document reuse for basic layout.
Download it again. It was just updated from "Nicholas Liggett" and "Falcon
9v1.1 - Reentry Nozzle Study" to "Elon Musk" and "Hyperloop Alpha"

------
tlb
I worry that the necessary straightness of the tube will be difficult to
achieve. The pads float 0.5 to 1.3 mm from the inside of the tube, so
deviations from straightness on that scale will cause the pads to crash. Any
contact at 760 mph would probably damage the pad, plugging the air holes.

~~~
bloaf
If someone shot the tube with a medium sized gun, would the impact cause an
internal defect on the order of 0.5mm?

~~~
conroy
The tubes for the passenger-only version would be 20 to 23 mm in thickness.
I'm not sure what damage a medium-sized gun would do to steel that thick.

~~~
notaddicted
According to some quick youtube searching, armor-piercing rounds versus
regular steel would create problems:
[http://youtu.be/q_zZd0NsYG4](http://youtu.be/q_zZd0NsYG4) .

~~~
bloaf
Sure, so would high explosives directly on the tube. I am considering a
situation that could conceivably be accidental because it is incredibly
difficult to engineer something that can stand up to malice.

~~~
jlgreco
The bar to malice is important to consider. If the bar is "high explosives or
shaped charges", then you are probably okay. If the bar is "a redneck who is
bored with shooting stop-signs", then that might be a problem.

~~~
glibgil
Bored people don't shoot guns at trains today. Why would they start?

~~~
kingkawn
Because there's no possibility that shooting at a train would result in a
fireball a thousand yards high, whereas...

~~~
count
How thick are propane tanker cars?

~~~
jlgreco
Back of the bar napkin calculation with Barlow's formula, assuming 200psi
(about what bbq tanks are at iirc?), somewhere around half an inch.

------
tlb
He doesn't properly address thermal expansion of the entire system. A long
straight steel tube from SF to LA will expand a total of 300 meters over a 40C
temperature change. Where does it go?

~~~
eco
How does rail address this?

~~~
tod222
Prior to the use of continuous welded rail there were gaps between each 39
foot section of rail.

Nowadays with welded rail, careful compliance with construction standards is
required. Slow orders are issued during heat waves and frequent track
inspections are conducted to look for potential or actual sun kinks.

A good summary of the issue can be found in the Heat Orders FAQ from Virginia
Railway Express. [1]

[1]
[http://www.vre.org/feedback/frequently_asked_questions/faq_h...](http://www.vre.org/feedback/frequently_asked_questions/faq_heat_orders.htm)

------
oskarth
Why is everyone so negative?

Musk just posted a innovative, open-source, thorough, alpha design study which
addresses a real problem.

Personally I think it's great, and I hope it gets taken seriously by the right
people.

------
rwmj
It's interesting that the Shinkansen has raised sections built on pylons,
especially the new bits they built in the last few years in the built-up areas
in the north of Japan.

~~~
nickpinkston
As does the Chinese system and for the same reasons Musk addresses.

------
darkmighty
When I was a kid I had a similar idea, a low pressure elevated concrete tube
transport system. In my idea cars would get rails underneath them and freely
enter/exit the tube at some key points. The cars would get a compressor to
help keep the not-so-large pressure differential at bay; all inspired by
evacuated tube documentaries.

In other words, this has been proposed several times before. However, there
are some key innovations here - namely the compressor/nozzle pair and the air
suspension system - which may indeed change the game and make this viable.

------
nakedrobot2
This could be considered a "next generation train". Elon Musk as we know likes
to consider things from "first principles". It looks like here, he has taken
the first principles of high-speed "rail" travel on land, and come up with
this.

I think our children might enjoy using it ;-)

------
adwf
Looks like a good idea, but the major glaring technical point that seems to be
missing is the issue of horizontal stability within the tube.

When you get very low friction, high speed and no tracks, you're going to
start having attitude problems without the right control surfaces available.
Don't want to be spinning passengers around the roof of that tunnel!

~~~
jessaustin
The train could be just very bottom-heavy.

~~~
adwf
Perhaps on a straight, but in the bends or slopes you'd still need some sort
of control. I'm visualising the kind of oscillations that luge or bobsled
riders get in the winter olympics when they come out of the corners.

High speed and low friction could mean a lot of potential lateral movement
that needs to be controlled somehow. Maybe releasing some of the stored
compressed gas in controlled bursts like used in various space endeavours.
That would have the added benefit of off-loading some of the potentially
dangerous steam.

~~~
jessaustin
I've never operated a bobsled, but if they're like other machines I've
operated with physical controls that have mass or that move in response to
things that have mass, then the oscillations are caused by the driver. Which
makes sense because the better bobsled drivers are observed not to suffer from
these oscillations as often as their competitors. (In fact we might say they
are "dampers" rather than "drivers".)

In the absence of any controls, we would expect the train to climb the side of
the tube on a corner, without ever reaching a "horizontal" orientation. When
exiting a corner, perhaps there would be a pendulum situation, but I'm
thinking that a train that wasn't round in cross-section would damp that
pretty quickly. If not, the system generates compressed air. That can be
released through nozzles that are oriented and controlled to dampen the
rolling.

But you're right, of course. A bottom-heavy train is just a more effective
pendulum.

------
zeckalpha
Nice font. But looks good so far.

EDIT: There's a bit of minimally technical details, but this is neither highly
polished (public-perception-focused) nor highly technical (engineer-focused).
Is it a good balance or would one of the extremes have been better?

~~~
keiferski
It says right at the beginning that the first section is an overview while the
second section is more technical...

~~~
zeckalpha
So? It needed a section with more polish than the first section for not HN,
and a more detailed technical section.

------
fludlight
Cool concept, but a 500 mile bridge? Elevated transportation routes are
expensive [1,2,3]. The utopian dreams tend to give way to an ugly reality
[4,5].

The $1 billion budget for land and rights of way also seems wildly optimistic
for razing a path through multiple heavily populated metropolitan areas with
astronomical real estate prices. And let's not underestimate the tenacity and
ingenuity of a determined opposition [6,7].

[1] The 3.2 mile, mostly elevated [1a] BART/OAK connector with dinky little
cable cars will cost $361mm for the capital construction alone[1b].

[1a]
[http://bart.gov/docs/oac/OAC%20Project%20Alignment%20Map%20S...](http://bart.gov/docs/oac/OAC%20Project%20Alignment%20Map%20Satellite.jpg)

[1b] (slide 36)
[http://bart.gov/docs/oac/Microsoft%20PowerPoint%20-%20OAC%20...](http://bart.gov/docs/oac/Microsoft%20PowerPoint%20-%20OAC%20December%2010%20Award%20-%20Final.pdf)

[2]
[http://www.nationalaffairs.com/doclib/20080528_197905505cost...](http://www.nationalaffairs.com/doclib/20080528_197905505costsbenefitsandthewestsidehighwayreginaherzlinger.pdf)

[3] [http://www-pam.usc.edu/volume2/v2i1a3s2.html](http://www-
pam.usc.edu/volume2/v2i1a3s2.html)

[4]
[http://www.youtube.com/watch?feature=player_detailpage&v=DkZ...](http://www.youtube.com/watch?feature=player_detailpage&v=DkZSQS7lpHE&t=163)

[5]
[http://en.wikipedia.org/wiki/Embarcadero_Freeway](http://en.wikipedia.org/wiki/Embarcadero_Freeway)

[6]
[http://en.wikipedia.org/wiki/Highway_revolts#California](http://en.wikipedia.org/wiki/Highway_revolts#California)

[7] [http://dc.streetsblog.org/2012/06/25/indianas-big-dig-
raises...](http://dc.streetsblog.org/2012/06/25/indianas-big-dig-raises-bar-
on-absurdly-wasteful-highway-boondoggles/) (this one is hilarious)

~~~
eru
In Singapore all short haul trains (called MRT) are either underground or on
Pylons. Seems to work out well enough there.

~~~
simonebrunozzi
Yes (I lived in Singapore for 2.5 years) but they go quite slow. Top speed of
the most recent, unmanned MRT (yellow line) is about 65 Km/h.

~~~
eru
Yes. I was just talking about pylons being viable.

------
nsxwolf
It's too bad the tubes and trains can't be made transparent. Imagine the view!

~~~
erikpukinskis
They should fly a helicopter down the California coast with a 4k camera
pointed out towards the ocean and then mount 30" LCD screens next to each seat
and play back the video at high speed, with image stabilization, correlated
with the position of the train.

~~~
thrill
Each passenger gets an Oculus Rift headset and can choose their virtual route.

------
magoon
This is supposed to be for commuters? I worry not many would commute daily
from LA to SF.

Also, travel time to the station has to be taken into account, even more so if
we compare it to driving.

~~~
noisy_boy
Yes they don't because its not feasible _yet_. If your residence/office is
within 30 mins from the stations at points of origin/exit, then with this,
it'll be about 1.5 hours end-to-end (I'm sure auxiliary services would spring
up to support this like shuttle buses etc). I know people who change buses,
trains and do some walking, chalking a total of that much right now everyday
while working and staying in the same city.

------
hop
I wonder what they have in mind for how stations will offload/onload capsules
onto the loop. That will be heck of an engineering problem to solve - ultra
high safety, pressurized environment, fast turnaround times...

My initial thought was they would have switches on the track like a normal
train and capsules would veer off. But I bet they would come to a stop on
there own and then get lifted vertically off the train and into a
decompression area where they'd cue up.

~~~
jessaustin
This is addressed in the document. Essentially, each station is a terminus, so
airlocks are transited at low speed. A turntable reorients trains for their
next trip. The only switching required takes place entirely within the
evacuated zone. (This is for when spurs are built to serve e.g. San Diego or
Las Vegas.)

------
jessaustin
Claustrophobes will not be eager to travel in this fashion.

~~~
Someone
Then use [http://www.bbc.com/future/story/20130812-planes-that-can-
pic...](http://www.bbc.com/future/story/20130812-planes-that-can-pick-up-
trains). Compared to that, this Hyperloop idea is best described as "No pilot.
Less space than a 747", by teenage boys.

On the other, more important hand, at firt view, this looks feasible to me
(taking into account who published it)

I do wonder about the "little noise" claim, though. That fan a few meters in
front of you that compresses air so that you can float on it will not be
noise-free. I also think those tubes could start resonating at times.

Finally: only about a thousand passengers per hour or, alternatively, going at
close to the speed of sound, on guaranteed collision course, with 30s
intervals?

~~~
poub
Oh the irony if you try to access the bbc link
[http://www.bbc.com/future/story/20130812-planes-that-can-
pic...](http://www.bbc.com/future/story/20130812-planes-that-can-pick-up-
trains) from the UK :

«BBC Future (international version) We're sorry but this site is not
accessible from the UK as it is part of our international service and is not
funded by the licence fee. It is run commercially by BBC Worldwide, a wholly-
owned subsidiary of the BBC, the profits made from it go back to BBC
programme-makers to help fund great new BBC programmes. »

~~~
Someone
For those in the same boat: the BBC article talks of a design where maglev
trains, starting in city centers, accelerate to a few hundreds km/h and then
get picked up by a huge flying wing that swoops on it like a bird of prey.
Later, much farther away, that flying wing will put the trains back on a
maglev track, without ever landing.

See also:
[http://www.core77.com/blog/transportation/flotspotting_u_of_...](http://www.core77.com/blog/transportation/flotspotting_u_of_glasgow_teams_horizon_modular_airplane_system_looks_even_better_than_the_clip-
air_25120.asp)

That truly is a toy for boys (as in "looks cool and dangerous") unlike the
Hyperloop ("it may or may not be dangerous, but you can't see that, so who
cares?")

------
vpuzzella
Cool stuff here, but I'd rather see them focus their efforts on moving cargo
rather than people.

Find a way to remove the cargo (trucks) from the commuter transportation
system (roads & highways) and you'll improve the lives of many more people.

Moving cargo is a lot easier than moving people because it doesn't complain or
need to go to the bathroom. It also won't be missed by most people if it's
destroyed.

------
sukaka
Could someone buy land in the potential path of this hyperloop then block
access or sell the rights or land to make a profit?

~~~
sczkid
No, because of the principle of eminent domain, meaning the government of
California can forcefully purchase the land for public use:
[http://en.wikipedia.org/wiki/Eminent_domain](http://en.wikipedia.org/wiki/Eminent_domain)

Legally, whoever tried this would be forced to give over the land for "fair
market value", and if they were difficult about it they'd probably receive
much less.

------
rossjudson
I was trying to figure out where I'd seen this before.

[http://www.thecomicking.net/2012/06/25/the-
core/](http://www.thecomicking.net/2012/06/25/the-core/)

The (terrible) movie "The Core" has a vehicle -- the Virgil.

------
realrocker
Anyone has the resources from this pdf and willing to commit to github? Lets
start forking!

~~~
triplepoint217
I sent a request to their feedback email seeing if they would be open to
something of that sort. Copy pasting formatting to Markdown/ReST wouldn't be
too bad, but it seems to me polite to ask permission.

------
Egregore
If this system will evolve, how switches will work between different routes?
Will it be only from station to station?

------
JulianMorrison
Eeeh, that looks awfully cramped. Especially for anyone a little overweight.

~~~
drgath
No more so than flying coach. It actually looks more like flying business
class on BA.

------
205guy
So what happened to section 5? Is that redacted by the NSA?

------
kingkawn
How would you rescue when pod breaks down mid tube?

~~~
drhodes
The pods have something similar to landing gear for use at speeds less than
100 mph. Still, what happens to pods that are sandwiched between two
malfunctioning/mangled pods? I think they should have contingencies where
sections of tubes may be excised safely.

------
sschueller
Isn't the pod going to be extremely loud?

------
xoail
Honestly, this paper should be made as a mandatory reading in schools.
Honoring modern science and economics.

------
bbrooks
What's the ideal format for an open source whitepaper? Certainly not PDF.
Straight up HTML on Github?

~~~
jessaustin
What disadvantages do you perceive for PDF?

~~~
mpyne
The same ones that we perceive for "open source development" on compiled
binaries.

With that said, it's not hard in this case to C&P to something more suitable
for GitHub and really get things kicked off, but LaTeX would be more suitable.

~~~
jessaustin
Although it was effectively the case before, since 2008 PDF is an open format.
It's quite misleading to compare it to a binary for which source is
unavailable. Since the format specifies compression, not every part of a PDF
file may be easily observed in a text editor, but there's a bunch of open
software that can read (and edit, and produce) PDF.

~~~
mpyne
Producing and reading PDF isn't the issue.

Again, we have open source tools that will generate, mangle, introspect,
disassemble, etc. to both ELF and PE binaries.

What PDF is _not_ is the "preferred format for development", no matter how
good the open-source tooling is.

~~~
jessaustin
Sometimes practical considerations preclude using the most "preferred format".
Will _editing_ this white paper really be a significant obstacle to deploying
a system like the one it describes? Conversely, does anyone _ever_ release a
white paper in LaTeX only?

~~~
mpyne
What are you on about?

No one here is saying that an open-source project to assist with Hyperloop
could not be launched. Nor that the ability to edit the whitepaper that was
released has anything to do with this proposed open-source project. Nor even
that people typically release whitepapers __only __in LaTeX format.

However if an open source project _will_ be launched with Hyperloop then its
success will be at least somewhat dependent on the ability of participants to
meaningfully contribute.

If "contribute" just means discussing key points then a simple Wiki might
suffice. However if "contribute" means technical designs, then the actual
drawings and data will be what's needed, not glorified screenshots of AutoCAD
and MATLAB.

~~~
jessaustin
mpyne you're hilarious.

------
tomrod
How does one maintain oxygen ratios?

------
andyl
So where is the 'loop'. Seems like this should be called 'hypertube'.

~~~
braum
the "loop" is likely at the end of each line so the pods can turn around.

~~~
escoz
did you read the paper?

~~~
braum
Not every word, I have a day job. If you have something specific to contribute
please share.

~~~
jessaustin
The proposal is to use turntables. Since emphasis is made in the paper on
maximizing turning radii, it could be that the train is not intended to be
_able_ to turn sharply enough to negotiate a feasible "loop".

