On Wed, 9 May 2007, Clive D. W. Feather wrote:

In article , Tom Anderson
writes

At Euston the two branches of the Northern Line are approximately at
right angles.

I was wondering, as i was sat on it this morning, whether it would be
possible to construct a connector between the Bank and CX branches
around about Euston, so that they could act as two arms of a loop, with
trains running Kennington - Bank - Euston - Charing Cross - Kennington
and vice versa. Based on what you say, perhaps not.

Not really, since it would have to miss Euston.

Not so! See my previous post with the ramble in it. Or not, if you prefer.

If so, though, it could be a useful way of focusing trains on the
termini-to-town section of the line, which i assume is the most heavily
loaded,

I'm not totally convinced it's enough busier, though there's a fair number
get off at KXSP northbound in the evening peak.

Facts, sir - pray do not trouble me with them!

either for extra trains during the peaks (which wouldn't need to negotiate
the delay-inducing Camden Town chicanery)

Why "delay-inducing"? The junctions are designed so that all possible
non-conflicting moves can be done simultaneously.

That's not enough to eliminate delay - you also have to be able to do
conflicting moves simultaneously .

It was my understanding that although the junction is graded, there's
still interference between the lines. Say you have a Charing Cross train
coming from Edgware, and a Charing Cross followed by a Bank coming from
Barnet. You can have the following sequence of events:

1. Edgware - CX arrives at platform
2. Barnet - CX arrives at platform
3. Barnet - Bank is approaching in tunnel behind
4. Edgware - CX departs
5. Barnet - CX is ready to depart, but has to wait for the Edgware - CX to
make it some way down the line before it can follow it, and is now delayed
6. Barnet - Bank is now stuck behind the waiting Barnet - CX, and is now
also delayed

So an Edgware - CX train has managed to delay a Barnet - Bank train.

It's the big-metal-rolly-thing analogue of head-of-line blocking in
network switches, if you've come across that (not that i'm implying you'd
sully your hands with such link-layer trivia ).

This wouldn't happen if there was enough space in the crossover tunnels to
hold a train, so that you could move it past the diverging junction,
allowing the train behind it to head off, before you were able to move it
over the converging junction. However, IIRC, posts passim established that
this was not the case for all the tunnels. It would also not happen if you
planned all arrivals so that this kind of conflict was avoided, but on a
line of this scale and frequency, in real-world conditions, this is
wishful thinking. Hence, the Northern line as we know it today.

It is entirely possible that i am seized of the incorrect extremity of the
stick here, but this is what i heard on the internets.

... it seems that it might be plausible.
[...]

I've now dug out more detailed material. The northbound CX branch under
Drummond Street is almost exactly in line with the southbound Bank branch
under Euston itself.

The Bank branch platforms are centered on Eversholt Street. The southbound is
under Doric Way and the northbound is 60m away, under the southern edge of
the main Euston station (almost mid-way between Doric Way and Grafton Place).
The CX platforms are at about 20 degrees to them. Their western end is under
Cardington Street and the eastern under under Euston, about 100m north of the
southern edge of the main building. The southbound Bank crosses under these
platforms about 40m from their western end at about 30 degrees to them; the
northbound crosses about 20m west of that end at about 45 degrees. The two
tunnels converge under the intersection of Coburg Street and Star Street.

The CX branch runs under Drummond Street to North Gower Street, then in a
curve radius 170m to meet Hampstead Road at the northern edge of Euston Road.
[The Victoria Line crosses Coburg Street 20m north of Euston Street and then
runs in a straight line to the point where Hampstead Road crosses the Euston
Road underpass.

Running east, the Bank branch southbound is straight for 220m from the
platforms to Ossulston Street, then curves for 80m to a point under the
middle of the northern edge of the British Library; the northbound leaves the
Euston Loop at the same point, crossing under the Victoria at Chalton Street,
40m south of the southbound. It then runs 90m east and crosses Midland Road
about 180m north of Euston Road; this is where the King's Cross Loop comes
off it. It's then 110m to the western end of the KXSP platforms, which is 20m
west of the east edge and 70m/50m north of the southern edge of St.Pancras
main building. The southbound platform and track is in a straight line to run
under Pentonville Road; the two platforms end about 80m west of York Road,
almost directly under the northbound Piccadilly. The northbound platform is
about 20m south of the southbound; the two tunnels converge about 160m east
of the station.

Superb info, thanks!

tom

--
Throwin' Lyle's liquor away is like pickin' a fight with a meat packing
plant! -- Ray Smuckles