Tom Anderson wrote to uk.transport.london on Sun, 22 May 2005:

Next question, then - how come? It has a pretty decent 28.5 tph, it's the
shortest proper line in the whole network, and all but one of its stations
are on other lines as well!

Northbound trains usually have a little spare capacity until they reach
Victoria, at which point they invariably fill fuller than is
comfortable! I don't know at what point you begin to be able to breathe
again, but I assume at Kings Cross!

For northbound trais to be be full at Victoria, Brixton, Stockwell,
Vauxhall and Pimlico would have to be generating as many passengers
as all the Northern line stations from Morden to Kennington put
together,

A great many people change on to the Vic at Stockwell, probably because
it is a great deal quicker across London than the Northern Line is, and
if you are going to (e.g.) King's Cross, it's enough quicker to make
changing worth while. You can usually get a seat at Brixton, unless a
train has been cancelled, but in peak hours it's often standing-room
only after Stockwell.
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/
Website updated 3 April 2005