The phenomenon of feeling pressure when we are
in a tunnel while travelling in a running train is known as piston effect.
Actually, the pressure in a
tunnel is generated due to pushing of air column when a train passes through it
especially at a very high speed.
Pressure fluctuations in high-speed rail
tunnels occur due to train induced pressure waves and by passing trains.
Pressure waves - compression or expansion waves - are generated when : (i)
trains enter or exit tunnels (ii) trains pass cross-sectional variations (iii) trains pass openings to the opposite
bore (for single track double bore tunnels) (iv) trains pass shafts or other openings to the
outside
Pressure waves in tunnels travel with the
speed of sound. If compression waves hit portals 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 and is due to drag effect.
