IN TWO YEARS’ time West Germany’s state railway will run the first sealed
trains in Europe since Lenin travelled from Switzerland to Russia in early
1917. The Deutsche Bundesbahn trains will have a top speed of between 250
and 280 kilometres per hour. This makes them the second fastest trains in
the world, and only slightly slower than the French Train a Grande Vitesse.
Unlike the TGV, however, the German trains will be sealed like an aircraft
to minimise the discomfort of pressure changes on passengers’ ears.
As the speed of trains increases the effects of pressure waves on the
design of tunnels are becoming more important, says Roger Gawthorpe, who
is head of aerodynamic research at British Rail’s test centre at Derby.
Lobbying by environmental groups is also forcing railways to lay a large
proportion of their new lines in tunnels. Gawthorpe says that pressure effects,
rather than the size of trains or the economics of construction, determined
the size of tunnels on Britain’s high-speed rail connection from London
to the Channel Tunnel.
Most passengers notice the change in pressure after a train enters a
tunnel, which is often quite startling. Besides the resistance, or aerodynamic
drag, that a moving vehicle experiences, the train also produces a compression
wave of high pressure as it pushes the air in front of it; and like all
air pressure waves, this one travels at the speed of sound. The train also
sucks the air behind it, producing an expansion wave of low pressure. This
wave follows the train into the tunnel and rapidly overtakes even the fastest
train. It is these waves,…
