We touched on the broader reasons for this back in 2012 in
A Brief History of Sidings. Ultimately there comes a point where capacity improvements stop being about obvious things, such as the quantity and quality of trains (and the signalling they use), and starts being about lots of smaller physical or logistical pinch points.
Getting drivers in the right place at the right time, for example, becomes increasingly difficult as frequencies increase. Because there inevitably comes a point where the gap between trains is
less than the time it takes to walk the length of the platform. This is not a problem for passengers, obviously, who can board at any point, but it instantly means that
the same driver who brought the train in cannot take it out again. Thus a level of logistical complexity is increased to staff rostering that didn’t exist before.
There are other, more physical limitations.
One of the reasons for the extensive works underway at Victoria, for example, is because as train frequency increases so does the speed at which you need to move alighting passengers off of the platforms (this is also why regular commuters often ignore the signage at Kings Cross – it is focused on getting infrequent travellers away from the platform areas quickly, rather than pointing regular passengers towards quick interchanges).
thought only metropolitan line stopped at aldgate
