Tom Anderson writes:
How many sizes of tube train are there?

Two major types, "tube" (deep tunnel) and "subsurface" (cut and cover),
as you surmise.

Central line trains always feel tiny compared to, say, circles; and
it makes sense that a deep tunnel might be smaller than a cut-and-cover;
is that so?

Yes, I just said so. Weren't you listening? :-)

Are northern/Victoria/Piccadilly any bigger or smaller?

No substantial difference. The standard tunnel is 11' 8.25" (3.56 m)
diameter on all these lines. Sections built since WW2 have slightly
larger tunnels, but for the purpose of reducing air friction rather
than making the trains larger.

Where car *lengths* are longer, the cars either have to be slightly
narrower or have a taper at the ends in order to fit around the curves.

How come metropolitan trains feel so much bigger than circles (as
big as real trains), when they use the same track? Are they bigger...?

Yes, slightly.

Is it because the northern half of the circle has bigger tunnels?

Yes, because it was built originally to take broad-gauge trains.
And the Metropolitan main line, of course, is pretty much all on
the surface.

There are actually three main flavors of subsurface train, called
"A", "C", and "D" stock, all slightly different sizes according to
which tunnels they run through.

Does that mean one could run real trains through them?

The entire subsurface system was originally designed for "real trains"
to be run through from the main line railways, and at one time there
were many track connections where trains did run through. These
included (listed from memory, but I should be pretty much correct):

* London & South Western - Turnham Green, Hammersmith
* Great Western - Ealing Broadway, Paddington
* West London - Kensington (Olympia) (modern name), Latimer Road
* Midland - St. Pancras
* Great Northern - King's Cross
* London Chatham & Dover - Farringdon
* Great Eastern - Shoreditch
* South Eastern - New Cross
* London Brighton & South Coast - New Cross Gate (modern name)

All these connections have been removed over the years, one way or
another. In some cases the through services onto another line have
ended; in two they have become the only service and the line has
changed hands. (Hammersmith to Turnham Green, now Underground;
King's Cross Thameslink [modern name] to Farringdon, now Thameslink).

Still other track connections were built for possible through services
but never used that way. There's also at least one case where the
"through" service was always effectively only the service on the line:
from about Putney Bridge to Wimbledon, the District runs on ex-L&SWR
track that the L&SWR itself never used much.

But clearances have changed over the years, and today main line trains
are generally a bit larger than Underground subsurface trains, so if
such through services were reinstated, they would probably have to use
the latter size of train.

Is there any way to get a train
from, say, the Anglia or Great Eastern lines near Liverpool St down into
the circle?

Not any more, but that's where one of those "track connnections built
for possible through services but never used that way" was. In fact,
for a little while while the Metropolitan (and Circle) line was being
built, its trains terminated at Liverpool Street main line station.

Does the Thameslink tunnel stop at Moorgate?

At Moorgate, the Thameslink tracks are side by side with the Metropolitan
(also used for Circle and H&C trains) in one tunnel. Originally this all
beloned to the Metropolitan company (and Moorgate station was then on
the surface). The combined tunnel narrows to 2-track width where the
Thameslink tracks end, so only the Metropolitan tracks continue.

Does that mean we
could turn the West Anglia into a Retropolitan (tm) line which connected
northeast London with the northern half of the circle?

No. The tracks from Moorgate to Finsbury Park are not connected to any
others at Moorgate, never have been, and it could not easily be done.
They are in deep tube, far below the Metropolitan/Circle and Thameslink
tracks. This line used to be part of the Underground, incidentally,
and was the only deep tube on the system that was built large enough
for main-line trains. However, for historical reasons it wasn't
actually used for them until about 75 years after it was built, when
it was transferred to British Rail.

Or take the the met out to Chingford?

If the junction at Liverpool Street was put back in, that could be done.
Of course it would require major work, since the main line station was
rebuilt without providing for it.

In general the reason that these most of these services were withdrawn
or never begun is that it was found more efficient to operate the lines
the way they now are. Long routes that are partly shared with other
trains are more subject to delays. (Similarly, some people feel that
the Circle Line should be withdrawn to allow the tracks to be used
more efficiently by the lines they are shared with, and so on.)

In addition, there are compatibility issues with different systems of
electric trains, plus diesels on the main lines. In the days when the
through connections I listed above were put in, the subsurface lines
of the Underground used steam locomotives -- today nothing but electric
trains would be acceptable.

And what's so light about the DLR? It's standard gauge, after all.

The trains are of lighter construction and would not be safe in a
collision with other trains. Light railways also get to economize
in other ways.

Could you in fact take one train and drive it willy-nilly over
the entire network?

The Underground has many incompatibilities between lines, including
not only differences in size but also in in signaling or automatic
train control systems, and even in the position of the live rails.
If you imagine all other trains being safely off the tracks so that
the usual safety rules didn't apply, *and* all the signal-related
systems being disabled, then I think it would be physically possible
for a Central Line train to be driven onto and over any of the other
lines you wanted. (To reach the Waterloo & City it would have to
be moved by crane, as that line has no track connections at all.)

For more information both about present and former track connections,
and about what rolling stock is normally allowed on other lines, see
Clive's Underground Line Guides http://www.davros.org/rail/culg.

is it obvious i spend far too much time in the bloody tube and
have now lost my mind?

Yep. Pleased to meetcha.
--
Mark Brader "You have a truly warped mind.
Toronto I admire that in a person."
-- Bill Davidsen

My text in this article is in the public domain.