One idea: Couldn't one simply use the many many expansion joints of the tubes to mount electric vacuum motors? If you rig a control system to evacuate air in as many sections of tube as are necessary for the pod to travel at a time (say 10 sections), you don't have to evacuate the entire tube, just segments. If you then do 10 sections at a time in front of the pod, it can just act as a buffer of vacuum around it as it is propelled through the tube. If the compressing segments fail, you still end up in a fail-safe situation, as the tube will then just be filled with air that will slow the pod or stop it entirely. Your worst case is an uncomfortable deceleration state for passengers, and the pod will sense this via a simple speedometer and tell the one behind it that it's slowed down so the other ones will do the same.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
One idea: Couldn't one simply use the many many expansion joints of the tubes to mount electric vacuum motors? If you rig a control system to evacuate air in as many sections of tube as are necessary for the pod to travel at a time (say 10 sections), you don't have to evacuate the entire tube, just segments. If you then do 10 sections at a time in front of the pod, it can just act as a buffer of vacuum around it as it is propelled through the tube. If the compressing segments fail, you still end up in a fail-safe situation, as the tube will then just be filled with air that will slow the pod or stop it entirely. Your worst case is an uncomfortable deceleration state for passengers, and the pod will sense this via a simple speedometer and tell the one behind it that it's slowed down so the other ones will do the same.