the big issue is that it then doesn't do anything for Northern Line relief which is one of the main aims! (Tooting Broadway for example has NR Fenchurch Street type foot fall.)
When the plans were first out, a sizeable chunk (if not a majority) of informed internet commentators were of the view that a CR2/Northern line interchange in SW London would be a net dump of more passengers onto the Northern line, due to it now becoming the quickest route between Wimbledon and London Bridge (and Bank if the station doesn't keep a Waterloo service).
My own view, after arguing back and forth for a few months, looking at journey times and psychology*, was there'd be a some relief of the Northern line - created by people from Balham/Tooting wanting the West End now taking CR2 rather than the Northern line, and that the number of people changing between lines at Balham/Tooting Broadway would effectively cancel each other out - and probably wouldn't be that many compared to those staying on.
The
SWIRL plan that the TPA has clearly borrowed heavily from, borrowed my idea for dealing with this complaint - namely
building a branch to the Northern line and diverting the Wimbledon terminators there (the crayons then really come out to serve Wimbledon with those trains, but we'll ignore that). With 10tph branching to Tooting/Balham, you get most of the abstractive effects to the bottom of the Northern line, but you don't get anyone coming off trains from Chessington, Epsom, Shepperton or Hampton Court and adding to Northern line crowding.
*how much time is staying sat in a seat rather than standing worth? how much time is having a cross-platform change, rather than a regular one worth? is spending less time on a train worth it if you have to walk further from TCR/Victoria/Euston than you would from Green Park, Oxford Circus, Leicester Square?
