JRS: In article , dated Thu, 23
Sep 2004 18:58:46, seen in news:uk.transport.london, James Christie
posted :

With difficulty, because GPS has an accuracy of +/- 100m, unless of
course you are using Differential GPS, but that is mainly a maritime
system.

Differential GPS with respect to a set on the platform (or station)
seems an obvious move - even deals properly with landslides.

But I believe that a platform is the only extended object which comes
that near to just under a train door; if so, use a pair of solid object
detectors below door level, one at each side of the doorway.

Sonar or radar could be used. ISTM likely that if a suitable
electromagnetic structure were fixed to platform edges (e.g. a
perforated metallic strip) then one might use resonant reflection, and
be platform-specific.

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