In article , Ralf Hermanns
writes
What are the PED built for?
Obviously, they are a safety increase for the passengers on a crowded
platform - but in a movie of a "jubilee line drivers view" I saw, the
speaker said something about the ventilation systems of the trains. He
emphasized that the ventilation was the primary reason for building them,
and safety came just along with that.

You've slightly misunderstood, but only slightly.

The primary purpose was to improve the airflow in the stations. When you
consider that tube trains almost fill the tunnel (though less so on the
JLE, where the tunnels are wider), a significant portion of the train's
power goes to pushing air down the tunnels in front of it and pulling
air into the tunnel behind it. If this airflow can be improved there is
the potential for significant cost savings, even after the additional
costs of the door mechanisms.

The doors also provide additional safety for passengers on the platform.
But this is an additional benefit that comes free, rather than being the
purpose behind them.

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Clive D.W. Feather, writing for himself | Home:
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