Here is the actual definition of a vacuum, just to establish some kind of definition so you can follow along:
http://www.engineeringtoolbox.com/vacuum-d_837.html
In your own Quora link, it talks about partially evacuating the tube to reduce air friction on the pod - the Quora link even calls it "vacuum" - but I wasn't the one who started debating this. You're the one who got upset when they misunderstood the whole underpinning concept of the hyperloop: reduced air density to allow to higher transportation velocities.
I assumed a reasonable 50% vacuum. It might be more, it might be less, but they certainly are not going to try to push any cars through this tube at an atmospheric standard of 15psi. Even if you could get the air out of the way fast enough to approach 500mph, the compression of the air going around the car through such a tight space would generate large amounts of heat that would need to be dealt with. It is air being compressed in front of returning space craft that creates the heating, not 'air friction'. So all that fan on the front of the car is going to do is help 'float' the car away from the walls of the tube and reduce the amount of electromagnetic energy from the periodic motors necessary to lift and move the car.
If you don't believe me about the air pressure, you can read more.
Air velocity in a vacuum, observe that all the charts are using logarithmic scales and not linear ones:
http://www.engineeringtoolbox.com/vacuum-pipes-air-velocity-d_1195.html
Basic compression of gases. Fun fact, diesel engines work by compressing air to the point where it is hot enough to ignite the fuel. Just compression, no spark. Adiabatic is what we care about in this topic:
http://www.engineeringtoolbox.com/compression-expansion-gases-d_605.html
Heating from atmospheric re-entry, which is really just an open-system, higher-velocity,
lower-pressure example of the physics that would be at play here:
https://www.quora.com/Why-does-a-spacecraft-heat-up-during-reentry
Now, that we've established that pulling a vacuum is a key aspect of a hyperloop system, lets discuss the mechanical implications of said vacuum. If you pull
ANY kind of vacuum on a sealed system, the weight of the air that is
outside of the system exerts a force upon the entire system. In this particular case, you end up with a case that can simply be described by equations for uniform loading of a beam:
http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html
But even uniform beam stress equations are simplistic explanations in this particular case. The entire hyperloop fits the definition of a thin-walled tube. If you want to calculate the strength of a thin wall tube under pressure, you use these equations:
http://www.engineeringtoolbox.com/stress-thin-walled-tube-d_948.html
You'll notice that the thin-wall equations have a coefficient of 4 multiplied by the wall thickness in the denominator, compared to a coefficient of 2 for the 'more simple' uniform beam loading equations. This means that, all thicknesses being the same (wall vs beam), any tube will have approximately half the strength of a beam made of the same material.
But wait, let's not forget about stress concentrations. Every cut they make, every bend, weld, joint, or any kind of change in geometry creates a place for mechanical stress to collect in a system. So if they cut holes for windows, they would only be introducing weaknesses. Nothing fatal, as long as they pay attention to exact geometries used: did they use a large enough radius for the edge chamfers, should they use a larger radius for the corners of the window? If you're curious:
http://engineersedge.com/material_science/stress_concentration_fundamentals_9902.htm
I'm not even going to bring up the dynamic stresses of the car traveling through the system - simply because I don't feel like doing/explaining 2nd and 3rd order Diff EQs or any kind of Laplace transform if we get into any kind of feedback & control territory. But off the top of my head, the hyperloop system will need to be designed to cope with how the air pressure in front and behind each car will play out across the tube, vibration of the super structure from turbulent and laminar flows, and any kind of wave motion of the air in the entire system.
Now, never mind the math. There is still the issue of actually building a system like this: sealing the windows will introduce additional operating costs by way of additional workload on the vacuum pumps (imperfections in workmanship leading to leaks), and additional construction costs by complicating the design and assembly. In theory, practice and theory are exactly the same, in practice, they are nothing alike
Of course, lets assume they solve all the engineering and construction problems: cutting windows still doesn't solve the strobing effect that those windows would create for anyone traveling past them at 500mph, even when you do cut them so large that they might be impossible/un-economical to make. So this might stop people from ridding the hyperloop at all in the first place. Not good for a fledgling system. But looking out a window still assumes you can make anything out when it wizzes by at 500mph and at close range. So I suspect that when the designers, and financiers, are faced with the choice of "$200 infotainment system for each car" or "multi-million dollar development and install costs for windows that increase the chance of system/project failure", they are going to pick the infotainment system each and every time. Plus, they can sell ads on the infotainment system to help recoup construction and operation costs, and they can't on the windows.
I never once suggested having windows was impossible; I even provided suggestions of how it could be done. No, I suggested that it didn't make economic sense to do so. Seems that Hyperloop One agrees with me, judging by their prototype. If they were considering windows for their design, they would have probably built at least one section with them to test things like bleed rates and stress on the system as cars pass through it.
tl;dr: applying a force evenly over an entire surface adds up quickly and "added up force" is just fancy engineer talk for "added up cost"- and if you're going to throw a hissy fit online when someone contradicts you, you should really know what you're talking about first.
As an aside, the NASA Space Power Facility is home to the world's strongest vacuum chamber at 10^-6 Torr; otherwise known as "not pure vacuum". It is close enough for practical tests on earth's surface, but it doesn't approach the approximate "1 hydrogen atom/cubic meter" that we observe outside of planetary magnetospheres:
https://facilities.grc.nasa.gov/spf/
