How could you travel from Boston to Madrid in just under 90 Minutes? seems impossible no? while it is impossible at the moment the technology to make such a trip feasable exists and could be used today to create an intercontinental train that could whisk you from Boston to Madrid in just under an hour and a half.
How is this possible by using the program I wrote last week we find that it is 3398.4192609241 miles from Boston, MA to Madrid, Spain. that means to reach Madrid in 90 minutes you would need to run a train at an average speed of 2400mph, the peak speed would probably equal twice that speed however I don't know how to calculate that so regard that as uninformed and probably totally wrong.
Adam, the voice inside my head tells me, your forgetting that once a train reaches 600 mph breaking the sound barrier you would create an intense shockwave that would in relatively short order cause the train tunnel to shatter.
Well no, or not in my warped sense of physics and space/time, let's look at the tagline for one of the greatest movies ever. James Cameron's Aliens. Since your probably all too lazy to go visit the site to read it here it is.

In space, no one can hear you scream.

Why is this Mr Cameron I ask? Mr Cameron promptly informs me he is not a phycisist and tells me to go to this question on some webpage

good, you all went there and educated yourselves right? lazy, lazy, lazy!
Basically it says that since there that sound is a reverberation of waves through a substance. Normally, we as human beings experience theses waves through air, however virtually any medium can carry sound waves, water is one that you have probably experienced.
So how can we avoid the sonic boom problem? Depressurize the tunnels and no only do you avoid them being blown apart by very nasty sonic booms but you get the added benefit of a virtually frictionless enviroment, allowing the train to run at unheard-of speeds.
If you incorperate maglev technology into this you have basically a (virtually) frictionless system, a train capable of propelling itself at many times the speed of sound and whisking you to the far corners of the earth. Maglev is baically a series of magnets laid along the track that raise the train up off the track and propel it forward. This system would require extensive use of superconducting magnets since during the peak power demand of the train when it will be traveling at it's fastest it will be 1700 miles from either coast.
I'm not going to go into the logistics of building such a system but pre-fabricated tunnel sections 1/2 mile or so long manufactured on dry land and then sunk 200-300 feet under water seem like a great idea.
First you start out with the main spur between boston and madrid then once that's complete you begin work on a section from Madrid to Paris (654.58660822326 miles) and a section from Boston to New York (187.90030326743 miles), then do a section from Paris to London (212.75444473218 miles) and New York to Washington, DC (207.35484157083 miles)
In time this system could be extended to places as far away as Hawaii, allowing you to transit from Hawaii to New York (4956.4175873782 miles) in just over two hours. Tokyo, Japan to New York (6734.5296157774 miles) would take under 3 hours, by contrast an airplane flight would take over 14 hours.
Sound like fantasy? They Laughed at Jules Verne Too