"I wouldn't want anybody following me ten seconds behind down a tunnel at "thousands of km/hr"."
If yours breaks down , the closing speed between the next train and yours is going to be small if the next one is just ten secs back behind . If it was ten minutes back , you would be slowing down for ten minutes while the next one caught up with you .
The air cushion pushing and pulling you forward in an evenly / closely packed tube is a safety feature . The train in front is sucking you along if its under power  and the train behind is pushing you forward if its under power .