"Clive D. W. Feather" wrote in message
...

The tunnels are all double-track, aren't they? Apart from the new
(short) links.

365s already run through single-track tunnels between Ally Pally and
Potters Bar.

Agreed. However, when the 319s were ordered they were originally designed
without the end doors. The design was required to be modified to allow for
passengers to be detrained in emergency, using front and rear end doors, in
the tunnels under St P/KX. Presumably any following stock will be subject to
the same requirement. I suspect that, as previous posters have suggested,
this may be to do with clearances in the tunnels. I'll have to trawl through
some 1988/1989 "Modern Railways" when I get the chance!