Found this useful info under Question Corner/Science & Tech. The Hindu
click here
**TUNNEL PRESSURE**
Why do we feel pressure when we are in a tunnel while travelling in a train?
Ans:
Pressure...
more... in a tunnel is generated due to pushing of air column when a train passes through it especially at a very high speed. The phenomenon is known as piston effect.
Pressure waves — compression or expansion waves — are generated when trains enter or exit tunnels, they pass cross-sectional variations or pass shafts or other openings. Pressure waves in tunnels travel with the speed of sound. If compression waves hit portals (end structures of tunnels) they are reflected as expansion waves and vice versa. In addition, trains cause pressure fluctuations in a tunnel due to a sharp pressure drop when a train nose passes and due to the pressure decrease along the train due to drag effect.
The superimposition of pressure waves, reflections and the static pressure decrease along the length of train as well as the pressure drop at the trains nose will cause rather complex pressure situations in the tunnel and hence inside the train. Additionally, any track gradient will cause changes of atmospheric pressure for passengers as well. For example, a height difference of 100 m will cause atmospheric pressure differences of approximately 1.2 kPa. The air flow loads play an important role in designing of front and rear profile of the trains and also electrical and mechanical equipment.
S.P.S. JAIN
Former Member, Engineering, Railway Board