Dave Arquati wrote:

And yet which station on the Picadilly Line is so over capacity that it
frequently has to limit passenger access and could really benefit from a
nearby alternative station?

The only feasible alternatives are Holborn and Leicester Square. As an
alternative to Covent Garden, Aldwych would be fairly useless, given the
slow access and egress, and its northbound orientation.

I also wouldn't separate point one (the effect on the rest of the line)
from point two. Trains that went to Aldwych wouldn't be able to go to
Covent Garden or Leicester Square, making overcrowding at those stations
worse. The solution to Covent Garden overcrowding is to increase capacity
at Covent Garden, not to reopen a station that would be of little use to
the majority of people heading to the area.

How serious are the suggestions in circulation for both a Waterloo-King's
Cross St. Pancras route and reviving the old Fleet Line plans for Charing
Cross to Ludgate Circus and beyond? The natural interchange for them is
Aldwych and the former project could make use of the branch tunnels,
although frankly at Holborn they'd either need a proper connection to the
Picadilly (I saw the other day that Aldwych was built with three lift shafts
and multiple exit routes from the platforms - yet another example of an over
elaborate station being uselessly inaccessible for through services) or else
build a new line to King's Cross St Pancras.