In message , Mike Bristow
writes
Suppose a driver became unwell, and required immediate relief. You
would need to:
a) get a replacment driver from whereever she is to the nearest
access point (read: from depot to station, or similar);
b) get the driver from the access point to the train (by, say,
discharging the current so it's safe to walk down the tunnel,
and then walk from the tunnel to the train).

There is a whole procedure in place for such an eventuality.

Basically this involves using the driver of the following train (either
in or out of service, depending upon where it happens) and repeating
this until an extra driver is made available.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)