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But those trains don't use the Hertford Loop (the line through Palmers
Green). The section from Alexandra Palace to Finsbury Park might be a
bottleneck, but aren't there extra tracks there?

There are three passenger tracks northbound and two southbound. This is
most obvious at Finsbury Park, where there are only two southbound
platforms. [There is also an Up Goods from south of Ally Pally, but even
if this was upgraded for passenger train use it would be pretty useless.
If you wanted fast trains to Dalston Kingsland, perhaps ....]

if it were run by TfL I'm sure there would be at least 6tph on each
branch (which they manage to run in the peaks, so there must be
capacity for it). The Piccadilly Line runs through similar areas, and
manages to justify something like 18tph.

Which may make more than 2/3tph off-peak on these lines uneconomic.

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