In message , Raymond Kirk
writes

I find it confusing! Can someone please explain it to me in
laymans terms.

The line continues beyond the platform ends at both terminii for two
reasons. During normal service hours they provide an over-run so that if
for any reason the normal stopping and safety procedures fail, trains
don't immediately run into a solid wall.

But their main purpose, certainly at Brixton (where the over-run tunnels
are 1200 feet in length) is to provide over-night stabling for trains so
that the service can begin with a reasonably frequent service in the
early morning without having to wait for trains to make their way from
the Northumberland Park depot.
--
Paul Terry