I'm sure that emergency friction braking could be implemented along with other aerodynamic measures to brake the pods at a rate probably only limited by the hum

I'm sure that emergency friction braking could be implemented along with other aerodynamic measures to brake the pods at a rate probably only limited by the human occupants. More importantly, (never say never, but...) this will not actually be a high-value target for terrorism; the individual pods don't hold enough people to make for a significant death toll, the tube itself being of a uniform construction would be relatively easy to repair, and the system itself doesn't really hold any political or cultural significance of note. Keep in mind that it would have to compete with far more valuable targets as well - the ever-popular White House, Pentagon, various national airlines as well as soft but high profile targets like shopping malls, stadiums, movie theaters. The risk of terrorism for this mode of transportation is actually less than many others.