In article , Roland Perry
writes
I've seen them doing maintenance of the Atlanta Marta (which is similar
to a Met/District Line style of urban commuter railway) late in the
evening by working on one track and having alternate-working on the
other. No doubt the UK's safety mafia would disallow this!!

Alternate working requires a sufficiency of crossovers to switch trains.
The tube lines don't have these, in general.

There's also a problem with trainstops - they need to be lowered to
prevent a train running the wrong way from "back-tripping". Except where
bidirectional working is allowed (e.g. at termini) this isn't designed
into the signalling.

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Clive D.W. Feather | Home:
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