What happens if something (earthquake / accident) breaks the tube? If we are flying along in that tube at 350 MPH and an earthquake ruptures the tube or a semi drives into the support beam and the tube breaks. What happens? obviously the tube will re-pressurize and I assume there will be a shock wave and that the capsules would then decelerate quickly. A capsule headed toward the break would obviously be totally obliterated. An airplane won't be affected at all. A train would be damaged, but not totally destroyed. History tells us that some train passengers would die, but most would survive. No one in a capsule that hits that break at 350 is going to live.
As disclosed in US patent 5,595,543 if a massive leak occurs that section of tube will be isolated from the network, and then flooded with air along the length of the tube. The rate and spacing of the air admission valves is such that the capsules are all slowed down by aerodynamic resistance, and the air between the capsules acts as a big air bag to help limit the force of any collisions. This represents less risk than a wing falling off of an airliner flying at 40,000 feet.
It may take a several hours to remove stranded capsules, and a few days to repair and pump down the tubes.