Tom Anderson wrote:

The Victoria line is essentially full at the moment.

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!

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, or the
Piccadilly line stations from Cockfosters to Caledonian Road. I don't know
those areas terribly well, and i realise that at least some of them are
very densely populated areas, but that seems quite surprising. Is it
because there are few people getting off the line before central London? I
guess a lot of Picc passengers switch to the Vic at Finsbury Park, and
perhaps Northern passengers to the Vic at Stockwell! Or is the Victoria
line picking up a lot of passengers from the suburban railway network?

I guess it could be a factor. If I were commuting up through Brixton to
Victoria and then needed the tube, I'd be inclined to get off at Brixton and
join the Victoria Line there rather than go through the squeeze at Victoria.
I'm not sure how fast the Balham - Stockwell route is but there could be
passengers using that as an alternative. Commuter trains at Vauxhaul have
often shed quite a few there at peak hours.