You're the first person to ever accuse me of giving too SHORT of an explanation! :) I don't have a thought out method in mind, but I'll try to cover a few possibilities.
To do an airlock, we would need a sealed chamber with a door for entry from the outside and another for transition to the tube. Both need to be air tight and the tx one might need to be extremely fast acting. The reason is that just like popping the mylar on the firing end of the ping-pong ball cannon, the air will rush in very fast and cause the car (ball) to accelerate the highest rate at the earliest time. In short, the doors need to be out of the way before the car shoots through them.
Alternatively, a longer airlock chamber could be used to give a buffer distance for the car to travel before it hits a door that hasn't fully opened yet. Of course, this needs to be calculated so the right amount of total air is ahead and behind the car at the time the door is fired. This is critical to the travel vacuum and to the air bunching at the slow-down stage. Either way, it's nothing more than a trade-off of top speed vs. vacuum pumping costs at the beginning vs. door opening speeds. Perhaps some 20 sided wedge-shaped door could be fashioned like the shutter on a camera where the mass of each piece is still small enough to move very quickly.
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