I'm going to take a stab and say, sort of guessing, that the train forcing air to flow with it through the tunnel reduces the pressure in accordance with Mr. Bernoulli's principles.
I think wfooshee's got it. As the air is forced into the tunnel its velocity increases, and so the pressure is lower in the tunnel is lower than outside. When you leave the tunnel the air pressure increases again, causing the popping in your eyes. It's like a mild form of the popping you experience in an airliner as it descends from high altitude.
It's quite a cool effect, I think I read somewhere that F1 cars can lose something like 20% of their downforce in the tunnel at Monaco due to the lower air pressure.
I'd guess the opposite, that there used to be a certain amount of air in a certain amount of space in the tunnel. Add the train and you've got less space for the air. Since the tunnel is long, the air can't get out easily and it causes a pressure increase in the tunnel.
I'd guess the opposite, that there used to be a certain amount of air in a certain amount of space in the tunnel. Add the train and you've got less space for the air. Since the tunnel is long, the air can't get out easily and it causes a pressure increase in the tunnel.
I started to say that, but I'm still going with the flow thing. I think (guess) you'd have a plug in front of the train being expelled from the end of the tunnel in front of the train, and low pressure sucking air in from behind. I think (guess) the train itself is in low pressure.
Somebody take a barometric altimeter on a train ride in the mountains!
Ok, here's my rather garbled understanding of what happens. I apologise in advance for any glaring errors I might make.
The equation linking air density, velocity and cross sectional area is "air density x velocity x cross-sectional area = a constant." As far as I know changes in air density can be ignored at speeds below mach 0.3 (~230mph), and so this equation essentially becomes "velocity x cross-sectional area = a constant." This means that velocity is inversely proportion to cross sectional area.
Bernoulli's principle states that any change in the velocity of a fluid is a result of a change in pressure - an increase in velocity is a result of a decrease in pressure, and vice versa. Because the tunnel has a comparatively small cross sectional area, and the air is being forced into it by train at whatever speed the air pressure in the tunnel drops. The faster the air the lower the pressure, so the difference in air pressure between inside and outside the tunnel is more apparent the faster the train travels. Faster train = bigger 'pop'.
That's my understanding at least. I've probably made several mistakes and a mockery of aerodynamics in general, but I think it gets across the point I'm trying to make. Kinda.
Happens here when I'm waiting for one of my friends thats driving the 5 Train. My ears seems to loose its hearing ability a little bit when a train is arriving. As soon as I or try to blow with my nose while holding my nose, (Like if you are sneezing.) I can hear better again. If I try to inhale with my nose while holding it, I hear a little bit less. Wierd...
