David Spiro wrote:
I don't know that this [express/local trains] was easier to
achieve due to the cut-and-cover method, or simply was a brilliant idea at
the time.

I'd class it a brilliant idea -- the other thing that express/local
achieves is much more operational flexibility (you don't get a total
line shutdown if a train breaks down) and the ability to run 24-hour
service (you can easily close down one track overnight to work on it).
But there's no doubt that building four tracks using cut and cover is
less than twice the cost of building two, whereas building four deep
tracks is probably more than twice the cost of two (once you've built
crossover caverns, step-plate junctions etc.)

The London service also seems to be more expansive in terms of its coverage
to local neighborhoods. There are too many places in the outer boroughs of
NYC where the only way to reach a subway is to first take a bus.

Same in London, even quite close in at times. The Bricklayers Arms/Old
Kent Road area springs to mind, but that's only because I used to live
near there. (In a perfect world, they'd extend the Bakerloo Line that
way.) In general, the gaps are increasingly being filled in by light
rail of various kinds -- the Old Kent Road area will get the
Cross-River tramway, for instance, assuming it ever happens. DLR is
expanding all the time, filling in a lot of East London near the river,
Croydon Tramlink has helped, the Uxbridge Road tram may happen
eventually, etc.