Dave Arquati wrote:

Whatever they can push through North Quay Junction, which is the
bottleneck for the entire network.

Frequency enhancement was considered through doubling of Bow Church
to Stratford and remodelling of North Quay and Royal Mint Street
junctions (to remove the conflict between northbound Stratfords
and southbound Canary Wharfs at the former, and between services
to/from Tower Gateway and eastbound Banks at the latter); however,
the cost was similar to the 3-car plan, but for only a 25% capacity
increase rather than 50%.

That implies that the network is already at the upper limits of
frequency in the peaks for any services passing through North Quay
or Royal Mint St. Curtailing some services has also been considered
(i.e having them terminate without passing through these junctions,
e.g. Poplar - Beckton) but was considered as highly inconvenient
to passengers.

Was this issue part of the reason why the takeover of the NLL was
considered, to provide a completely separate route on the
Stratford-Woolwich axis that doesn't require more trains to use the
existing single-track branch into platform 4?