On draught

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Unlike the Tube’s single bore tunnels, in much of the Paris Metro trains operate in wide two-directional tunnels

Sir, Dr Zuck (letter,
Jan 11) only attributes the ventilation characteristics of London’s Tube
lines to their narrow bore. The “piston” draught effect is a function of
train speed and the ratio of the cross-sectional areas of the train and the
tunnel. Unlike the Tube’s single bore tunnels, in much of the Paris Metro
trains operate in wide two-directional tunnels, so allowing much displaced
air to escape into the void above the neighbouring track. This is also the
case in London’s “sub surface” (eg, the District) lines. This piston effect
is a mixed blessing and can be problematic at high