On 26 May, 14:02, Paul Terry wrote:
Deep enough to avoid all of the sub-surface structures (foundations,
tube tunnels, etc) that are in the way of Crossrail. The tiny Post
Office railway was able to skirt round these, but that's not possible
for Crossrail (see below) - and, of course, there are many more tall
buildings now than there were when the P.O. railway was built.
Looking at the detailed planning briefs for crossrail, it shows the
Post Office railway tunnels as well. They don't appear to skirt round
anything - they take a fairly direct route. And while its true that
there are more tall buildings now than when the P.O. railway was
built, the buildings actually on its route don't get much taller than
Mount Pleasant.

the Post Office Railway doesn't have a straight enough alignment - it runs
north of Oxford Street, curving up to Wimpole Street and then coming
back south before the big loop up to Mount Pleasant.
Straight enough for what?
Ten-carriage trains of mainline proportions travelling at up to 100kph
through the tunnels.
(a) Why do they have to have ten carriages? What's wrong with more but
shorter trains?
(b) 100kph when they have to stop at stations every 500 yards or so is
absurd.

Yes, but Crossrail is nothing like a tube line - it is for mainline
services travelling at nearly three times the speed of tube trains in
the tunnels (and up to 160kph on the surface sections).
I don't see that as convincing rational. There's nothing saying its
compulsory for any cross-london relief for the central line to be
built for mainline trains. And the speed of tube trains in central
london is around 15mph, so you're talking about a tunnel that can cope
with just 45mph.

It doesn't need to hug oxford street when its not at a station, not
that the current Crossrail's Hanover Square and Dean Street Stations
are on Oxford Street either.

No, it doesn't need to hug Oxford Street (in fact, it runs slightly
south of the Central line), but it does have to be relatively straight
to achieve the anticipated speeds. Incidentally, there's no station at
Hanover Square - it is simply the eastern ticket hall for Bond Street
station
(a) Its a station
(b) Its at Hanover Square

Do you know how to put those two facts together in a meaningful way?

Crossrail is not really comparable with a tube service, though.
That's an absurd, rather circular, claim.