[ET3]

Pressuring a system (depending on the pressure) would be more stress and risk than a vacuum. Depending on materials used (concrete for instance - that prefers to be loaded in compression), the vacuum can actually IMPROVE stability and safety. As far as a pressurized "loop" as you describe, it would greatly INCREASE DRAG compared to a vehicle in the open air. What you describe is a 'Pneumatic Tube Transport' (PTT) system (as used in bank drive-up windows). PTT does not scale up well due to the enormous drag of air on the ENTIRE inside surface of the tube. Large PTT systems take a lot of power regardless of use. The energy per vehicle would be greater (but could be reduced a little by using hydrogen or helium that has less drag than air (mostly nitrogen)); however small molecules leak much more than large ones, and the expensive working fluid would have to be replaced.

Google "Evacuated Tube Transport" (ETT) or "Evacuated Tube Transport Technologies" (ET3) (trade marks of ET3 Global Alliance, and et3.com Inc.) on google and youtube -- watch the videos to learn of more advantages. Read the first US patent (5,595,543) to learn exactly how all of what Musk claims (and much more) can be accomplished. NOTE: ET3 (tm) is the product of over 200 experts, companies, and institutions in 18 countries. We hope musk joins our efforts to implement ET3. ET3 is optimized to accommodate BOTH cargo and passengers in car sized vehicles. Initial (local) routes will be at 300-400mph, and ET3 will accomplish over 50 times more transportation (ton-miles and passenger-miles) per KWh than the most efficient electric car or train. ET3 cost is less than a tenth as much as HSR (high speed rail), mostly due to much less vehicle mass It takes 1/30th as much concrete and steel to support 1200lb car sized capsules (400lb vehicles that haul the same 800lb payload as a typical automobile) than to support 100 ton locomotives on elevated structure. ET3 has over 10 times more capacity than HSR at 350mph design speed, and 0ver 100 times more at 4000mph. Refs: search “ doi: 10.3969/j.issn.2095-087X.2011.01.007 “ for a peer reviewed paper published in Journal of Modern Transportation; US patent 5,595,543 (many additional patents in the ET3 portfolio)