Pressure in a tunnel When entering a railway tunnel in a train I felt symptoms of overpressure (in my ears and sometimes eyes) many times. What is the physical principle of such overpressure? Clearly, there is a mass of rock and soil pressuring on the tunnel ceiling and walls but the tunnel is not sealed so one could expect pressure compensation by an airflow.
 A: You can use the time interval between the repetitions to predict the length of the tunnel. 
Below is a recording of the pressure in a train in a tunnel. A and B are the gates of the tunnel. Orange indicates ear discomfort. The pressure clearly oscillates. The frequency of the oscillation is the fundamental frequency of the tunnel. The tunnel is an open-end air column, so the frequency is f = v/(2L), where v is the speed of sound, and L is the length of the tunnel (or less, if there are ventilation shafts).

A: The mass (rocks) above the tunnel does not increase the pressure in the tunnel as the air in the tunnel is not carrying the rocks above the tunnel. I think the pressure increase is the result of the pressure wave that is created by the train moving into the tunnel. The train displaces lots of air but the air cannot leave the tunnel fast enough. Thus the pressure builds up.  
A: Air has mass and thus inertia, the tendency to remain stationary or in motion unless another force acts on it. Most tunnels are not much wider than the train that fits into it. If a tunnel is long enough, then the air inside it is fairly massive and thus has a considerable inertia. When a train enters a tunnel at high speed, it tries to push the air out of the other end of the tunnel, but because the tunnel is long and the air is massive, the air compresses against the mass of the air in front of it. This is the sudden pressure change your ears feel. The mass of the earth above the tunnel has nothing to do with it
