Pressure under both ears when outside?

How much pressure increase one can experience when a Train is travelling through a sufficiently long tunnel? What is the limit of pressure at which ears can sense the increase? Do train/tunnel designers take this pressure increase into account and employ pressure relief mechanisms?

• I guess there would be a correlation between speed of train versus how pressure wave propagates through the tunnel aka speed of sound.

• Answer:

  Normally in the open air, the train pushes the air in front of it along with it. In this case, air can move anywhere except into the ground but when a train enters a tunnel the air can't simply be pushed aside, it must rush backward through the narrow space between the train and the tunnel. This speeding air creates a suction on the train, just like opening the window of a speeding car can suck out maps and papers.This lowers air pressure inside the train, which can make the passengers' ears go pop.For the small tunnels, engineers generally don't think much but for large ones, sometimes the tunnels are made wider and sometimes ventilation shafts are also made.An example of best engineering is done in The most famous tunnel which is the thirty mile long named "chunnel" between England and France. Thirty miles of air is a lot to push around, so engineers had come up with a clever solution.The Chunnel is actually two separate tunnels, connected by cross passages. A Chunnel train doesn’t have to squeeze all that air backward between the train and tunnel wall. Those cross passages allow the air to escape sideways–into the second tunnel–greatly improving the air pressure inside the train.In 1994, the http://en.wikipedia.org/wiki/American_Society_of_Civil_Engineers elected the tunnel as one of the seven modern http://en.wikipedia.org/wiki/Wonders_of_the_World.Also, A pressure of around 7 pounds per square inch is what human ear can bear and human ear is very sensitive to the pressure increase, it can sense the pressure increase of one billionth of atmospheric pressure.