[On building 4-track lines]
It's most definitely a better way of doing things, but prohibitively
expensive when drilling tunnels. With cut-and-cover, you just make
your trench a bit wider.

However, this also requires the *street* to be wide enough. Common
in New York, not so common in London.

With deep-level tunnels, you have to dig twice as many tunnels.

Or make them wide enough for two tracks each instead of one.
(Of course that could also have been done with the 2-track lines...)

The trouble with a double-track tunnel is that you either have to
excavate a lot of wasted space above and below the tracks, or else
make the tunnel walls stronger to compensate for the weaker non-circular
cross-section.

But, after all, there are lots of them on main-line railways; it's not
an impossible choice. And "wasting" space above the tracks actually
would have brought some benefits -- better ventilation, reduced air
resistance, more space for any wires or other utilities needed in the
tunnel, and and the possibility of converting to an overhead power supply
if this was ever found desirable.

Most New York 4-track lines have all the tracks side by side, but not
all. There are multiple places where two tracks are above the other
two -- either using a double-deck cut-and-cover tunnel, or using a
cut-and-cover tunnel for the locals above a deep tube for the express.

That last should ring a bell -- around 1900 the District Railway was
planning a deep-level express line under their existing route from
just east of Earl's Court as far as Mansion House. This is the origin
of the section of the Piccadilly Line that runs below the District
from West Kensington to South Kensington, although of course it was
not built as an express route, bypassing only the former station.
--
Mark Brader, Toronto "Information! ... We want information!"
-- The Prisoner

My text in this article is in the public domain.