To run the whole service reliably (and it's designed for up to 18 or even
20
tph ultimately) with adequate turnround times and margins you'd need a
four platform station at Highbury, and there isn't room.

I was just wondering what the factors are that makes Highbury different from
other two platform terminals with higher service levels, e.g. Brixton on the
Victoria Line. That's a genuine question by the way, I am sure they are
many and various but I don't really know what they are. Paul has already
suggested that something as simple as the distance to the scissors crossover
would be one factor (assuming it couldn't have been placed somewhere else).
I wouldn't be surprised if something like passengers tending to wait around
on the platforms for a direct train instead of taking the first one and then
waiting further down the line would be a factor too.

G.