Let me clarify. The tunnels don’t go through the vaults, they go in between. That’s what all the switchbacks are - tunnels had to go around existing infrastructure. It’s the same with pretty much any line within the Zone 1 area. All that twisting and turning is the train following the line of the streets above. They couldn’t just go in straight lines because of the danger to any buildings above.
Wait a minute. There are actual switchbacks in the Tube, with two wyes and the train reversing on the middle segment? Like this one in Australia?
Nope.
I think it’s lost in translation.
“Switchback” is a North American expression, IIRC.
I’m going to use Australian railway terminology here, sorry, but hopefully it’ll be clear enough: the things described in the NYC subway and the LU are simply called “crossovers”. Trains transfer to another track without changing direction.
A “switchback” (or “Zig Zag” here) is used to aid in changes of altitude on mountainsides. The crossovers in the underground are just to change from one line to another. There are only a handful of zig zags in the world.
Oh, ha! That took a while for me to remember!
mm
My response to the OP would be something like “It’s uniquely old, and for half a century or so it was dreadfully underfunded while they kept cramming more and more people into it”. So now it’s running at some horrific percentage of design capacity while they frantically try to patch up the bits that should have been fixed years ago, and modernise it, and increase capacity, and try not to bring London grinding to a halt, all at once. Which is a bit of a challenge.
I seem to remember reading somewhere that the very tight fit between train and tube (caused by limited boring capacity with the old machines) was actually used to drive the ventilation - the trains act like a piston and push air through the tunnels, sucking it down some vents and blowing it out others. This is is a very smart thing but can make tube fires very very scary (you get a blast furnace effect) and also means that when the trains have to stop for some reason it can quickly get very very hot and stuffy. Is any of this correct or have the papers been making stuff up again?
Ah I think we did indeed have a mix up of terminology! When I used the term switchback I was just referring to the way the tunnel has to change direction to avoid existing structures. There are zig-zags or chicanes down there if that’s a more understandable way of putting it.
As TheLoadedDog has pointed out we have crossovers which permit a train to change from one side to the other, for example, northbound to southbound. These are heavily signalled areas as you obviously have to keep all other trains out of the way while undertaking this move. It’s not quite accurate to say that the crossovers are just to change from one line to another - if one moves from the northbound to the southbound then one has to change ends and take the train south. Although wrong direction moves are catered for they are extremely rare and involve a lot of personnel and paperwork to ensure it is safely done (oddly, on the tracks LUL share with National Rail there is no requirement for any of this and we can just drive the wrong way under verbal instructions from the signaller).
That’s what I thought. We call them crossovers as well.
And the Lithgow zig zag no longer carries trains, though the main line west of Sydney still goes near the zig zag. Within New South Wales, there’s another device to elevate a railway line in a short distance: the Border Loop north of Kyogle, which still is on the main line between Sydney and Brisbane. (And I can’t find any good pictures of it through Google, which doesn’t surprise me, since I driven through the area around the Border Loop, and it;s hard to find a good place to take a picture showing the loopiness of the line).
I suspect that there are no loops like that on the London Underground: the loop at Heathrow Airport doesn’t count, because it’s to help trains turn around, not to change elevation.
If you design them dirty, you never have to clean them.
Hmmm. According to Wikipedia (yeah, I know), there’s several reasons for the curves. As well as the bank vaults, it was to do with getting permission to tunnel and/or avoiding paying landowners.
er…yes…you see where I said they had to avoid existing infrastructure? I didn’t just mean the vaults. 
There is a *lot * underground. Cellars, utility pipes, rivers, graveyards, bank vaults. And that’s before even beginning to think of the things we’ve put down there since the 19th-ish century. Before any line was built the railway company had to obtain an Act of Parliament to grant powers. For most lines several Acts were granted as lines extended or changed route. For some lines, Acts for permission to extend were granted very early and maintained until very recently - no actual tunneling was done due to a variety of problems but the intent was always to extend. That has pretty much ended due to the now prohibitive costs of tunneling.
I met an interesting guy at Christmastime. He is a civil engineer in London and is involved in the demolition and rebuilding of several projects. He told me that he has incredible difficulties working with LUL because they simply don’t know exactly where all the tunnels are. He and they are working from maps draughted over a century ago and there are no truly accurate measurements available. And if he knocks down or constructs a building and the ground shifts as a result that could cause serious issues for underground tunnels.
I read a really fascinating book a couple of years ago called London Under London, describing all these things. One of my favorite things to read about is interesting bits of London, especially underground. 
Excuse me while I cough and splutter, and remark, “HOLY SHIT!” 
There’s a general consensus amongst Australian railwaymen that British safeworking practice is… well… a little bit, shall we say, loose in some areas. Either that, or our lot is a bunch of safety Nazis. Pom signallers tend to happily slot trains in close enough (I seem to recall something about 183 yard rolling headways or something) that would have an Australian signaller sacked and an inquiry launched. If a signaller simply verbally authorised a “wrong road” movement like that, he’d probably never work in the industry again. We do have some areas of bi-directional signalling which work as you’ve described, but those are pretty limited. In general, setting up single line working is such a headache that they tend to just close the entire line down and ring up the local bus company.
From a driver mate of mine, to give an idea:
Interesting viewpoint. I wonder if an Aussie signaller grade is directly comparable to a UK one. A UK signaller is not merely setting the signals but is in charge of the line. The signaller is god and if he tells you to set your train on fire then you go and ask the customers if they have a box of matches.
Wrong direction moves are extremely rare. They only take place if things have gone very wrong somewhere and (on my line at least) the signaller will prefer to have a train wait rather than do a WDM. Not sure what you mean by slotting trains in. Two trains cannot occupy the same signal section. Fin. End of story. Actually not quite true because in an emergency a second train could be instructed to trip past the signal and approach another train to assist the other driver/begin a pushout/detrain because of emergency or similar. But otherwise all trains are at least one signal section back. 183 yards? No way!
In general single line working is avoided here - we are fortunate that the bus companies are linked to the underground companies so rail replacement buses are not a problem. I think the Jubilee Line did do SLW sometime last year but mostly that seemed to be just to prove that it was possible.
I’m curious about how Aussie WDM’s work. From what you and your friend say I take it that it’s similar to LUL practice with paperwork required to be signed by a relevant operating official and the driver. How does that work in practice if the train is miles from where the signaller is? On our National Rail sections we can obtain permission over a secure phoneline (**not ** over train radio) and with the track I work on there are frequent signals and phones but presumably in Oz there are greater distances between SPTs and less contact with the signaller?
I’d have to go back and check. There are different grades involved - some guys just play with signals at a low level, and others are God.
I’m rather relieved to hear you say that. It’s sounding more and more like what I heard was bollocks, then. Yeah, we have a similar thing here on some closely-signalled sections (esp. the underground lines). The double light signalling will normally go from a danger aspect (red over red) to caution (green over red) to medium caution (green over yellow) to clear (green over green), but in closely-signalled areas there is a fifth aspect called “close up”, which is a red over red danger aspect, but a third light (a small green one) is illuminated below, and provided the speed is kept very low (20kmh, IIRC), you can proceed without tripping, and basically stop right on the arse end of the train in front (they try to avoid this though).
Sounds the same as here.
Sounds similar to here. I’d need to get back to you on what happens over long distances (though this is going to get worse with the closure of country signal boxes and everything being transferred to two or three locations for an area several times the size of the UK. Yes, it IS stupid, IMHO). I’m pretty sure they can get by though, because most dual track territory isn’t too far from civilisation, so they could probably get somebody down there before long.
Forgot to mention, there is some regular SLW authorised late at night because the crossovers need to be used once every 72 hours (I think it is) to remain certified for use WRT track circuitry etc. These movements are well planned for though, and if they don’t do them, they need to get the engineers out there to sign off on the crossover as fit for traffic.
I’m not sure of the timescales but we have similar rules regarding points. Although we’d never do SLW just to test points. Aside from anything else, our system shuts down for four hours each night to permit maintenance. With Underground we just schedule in crossovers to the early morning timetable. So the 5am train will go from the depot down to a certain station and then reverse over the points and back up to the starting point. I have no idea how that works for national rail - presumably they must also test the points regularly although given that they don’t require any downtime for maintenance they are presumably also doing it at night.
Aussie signalling sounds hellishly complicated. From my point of view if it’s not pure green or pure yellow then I’m not moving!
I suppose it is a bit, but it all makes sense when you’re used to it. Of course, what I’ve told you only applies to New South Wales, and it’s different again once you cross state borders.
Basically, in country areas, we’ve got single light signalling that would probably be familiar to you. The double light stuff in the metro and surrounding areas works on the same rules, but just provides extra information, eg caution has two aspects rather than one, so a green over red will tell you “section ahead is clear, but signal beyond may be at danger” and green over yellow will tell you, “section ahead is clear, but there may be a signal at danger within braking distance”, which in the real world usually means the third signal is at danger. Yellow over yellow is a standard turnout aspect for the diverging road, and yellow over red is diverging road with signal beyond possibly at danger.
I won’t even start on what those weird Victorians do…
But the first principle of Buddhism is still “every man for himself”
Mornington Crescent
Ah I get you. It sounds similar to National Rail signalling although there’s never a red involved unless you want the train to stop (shunting moves aside). Depending on how many colour lights you have (up to four I think) you have signals detailing how many sections ahead are clear. Many tube drivers have to learn both Underground signalling and National Rail signalling because lines extend outside of LUL. The signalling for NR and the signalling your trains have makes sense because of the speed of the trains - our little chuffers will perhaps do 70mph maximum! And the ones I am on will rarely do more than 50.
I forgot to reply to **Dangermom ** earlier. There’s a great book called London’s Disused Underground Stations by J E Connor that has a lot of information about the ghost stations. There are some pretty good websites on the subject too.
Sorry but you can’t breach Emslie’s Subtractional Allegory with a Sideways Scnickelgrube on a Saturday like that so I’ll have to set you back to Euston Square