Tom Anderson wrote:
On Mon, 30 Jul 2007, David of Broadway wrote:

The statements about your lack of express services were probably
referring to the Underground, where they're largely accurate, except
on the western Piccadilly and Metropolitan.

Strictly speaking, that're true, but my point was that NR trains act as
expresses for LU lines in some situations. For example, the Great
Northern from King's Cross, which only has stations north of Finsbury
Park (if you forget about Moorgate and all that) is the express service
of the northeastern Piccadilly. The London, Tilbury and Southend line is
the express service of the eastern District. Other lines don't have such
close correspondence to LU lines, but often serve overlapping areas at
the edge of town, providing a quicker service in.

And we in New York have the LIRR between Jamaica and Penn Station and
Metro-North between various points in the Bronx and Grand Central.
(Granted, the subway has substantially lower fares.)

In New York, I might hop on a 1 local train at 116th Street, transfer to
the 2/3 express at 96th Street, transfer back to the 1 local at 14th
Street, and get off at Houston Street. (Whether I save any time in the
process is a different question - in my experience, depending on the
time of day, it could jump me ahead one or two locals. OTOH, if there's
a long wait for the express, I might not even catch the local I started on.)

Or maybe I'm taking a relatively short trip, one for which the time
savings on the express are minimal. I can simply take whichever train
comes first, since the local and express stop at the same station,
usually at the same platform.

Or if a train breaks down on one track, the following trains can be
rerouted around it on the other track. The resulting congestion is
sometimes painful, and local passengers may have to backtrack, but at
least the trains can keep moving.

And, as has been pointed out elsewhere, parallel tracks make track work
much easier to carry out while the trains are still running.
--
David of Broadway
New York, NY, USA