Commenter David M on what rivers teach about transit:
It's interesting to note that in London the newest Underground lines have no branches (Victoria, Jubilee). In fact, when Jubilee was originally opened it took over one of the Bakerloo Lines branches, reducing the Bakerloo to a branchless line also.
For real complications, look at Camden Town [top center] on the Northern Line [black on this classic map] in London, England. Just south of this station is a complex deep underground junction that lets trains from any two of the branches south of Camden to simultaneously run on any two of the branches to the north. It is a marvel of engineering, but it is also an operational nightmare with trains run from any branch to any branch – one train runs late and it can cause problems on all of the branches.
London has wanted to simplify the operations by spliting the line into two and requiring an interchange at Camden Town. There are four platforms at Camden Town but the interchange passages are insufficient to handle the expected interchange traffic – so for now, it is cheaper to suck it up and deal with the operational issues.
There is an interesting effect of this interchange. Going south, both branches serve Euston Station before heading off to cross London on two different lines serving different areas of the core. You can get on one train at Camden, stop at Mornington Crescent and at Euston. You could get on the following train at Camden and arrive at Euston without passing through Mornington Crescent. The reason is that Mornington Crescent is on only one of the two branches, the other just bypasses the station. It makes for fun time when trying to get to Mornington Crescent.
The other night a Sydney rail expert was telling me that when the North West line is built, creating a four-way junction at Epping similar to the one at Camden Town, they will spend a number extra millions on the tunnelling to create the ability to route trains from any segment to any other. A similar decision has already been made about a similar junction at Glenfield in Sydney's southwest. I wonder how much could be saved if we let lines cross without connecting track, and required connections, where that pattern makes sense as part of a larger grid. It's not the right answer everywhere, certainly, but it sounds like London transit experts aren't very appreciative of all the flexibility that their great-grandparents gave them with the design of the Northern Line.
Even if you don’t use them in regular service, multi-way connections like these are very useful in cases of service disruption. On the IND lines in Manhattan, the MTA often deals with construction by routing 8th Ave trains onto 6th Ave or vice versa. In other parts of the system without such flexibility, construction can lead to long stretches being bustituted when even a very localised construction project prevents the line from running through.
This sort of flexibility also allows the system to easily adapt to changing ridership patterns. For example, during the 1980s, Brooklyn’s Brighton line was served only by trains to Nassau St or 6th Ave in Manhattan, but the rebirth of Times Square and other points on Broadway led to changes in passenger demand, and now most service on the Brighton line is provided by the Q train to Broadway. The extreme flexibility of DeKalb junction made this change possible. Washington DC would probably be better off today if it had constructed short connecting curves to allow some Orange line trains to turn right before Rosslyn and run into downtown via Arlington Cemetery and the Yellow Line bridge. These would not have been necessary when the system opened, but the densification of the Ballston corridor has filled the Orange line to capacity.
Complicated grade-separated junctions can be expensive and are worth a cost-benefit analysis, but there definitely are benefits.
Anon256: for occasional diversions, you can just have a simple diamond crossover, rather than the fancy grade-separated junctions at Camden Town or West 4th. And at West 4th, the flexible layout means that the “through” routes for the southbound F and northbound C/E actually have to take a diverging route through a switch, with all the increased wear on the switch points that that implies.
Jarrett: I think the real reason behind building in this flexibility is not to be able to do something like the Northern Line where there’s trains from every branch to every other branch, but rather to ensure it’s possible to switch a service pattern of A-B and C-D to A-D and C-B if some years or decades in the future, the relative demand on the branches means that that’s the more effective way to run the service.
There definitely are benefits to flying junctions, but on the IND, the costs outweighed them. The First System went way over budget and behind schedule, which together with the Depression is why they didn’t start building the Second System right away.
What I’ve read of the Northern Line is that the plan is to go to a configuration requiring a transfer at Camden Town, which would increase capacity.
I wonder if that quirk of Mornington Crescent is the cause for the game of the same name…
Slightly off-topic, but at Euston, the branch which goes west is actually on the eastern side (and vice versa)! This is a good exmmaple of why topographic maps (like the Tube’s) can be superior to geographic accurate maps.
The problem of branching halving frequency was solved for the Bakerloo Line when the Jubilee Line opened incorporating the Bakerloo’s Stanmore Branch. This enabled the frequency on the remaining Queen’s Park (via the mainline terminals of Marylebone and Paddington) t be doubled. Service on the Queens Park was also projected along the Network Rail Watford ‘DC’ line to Harrow & Wealdstone and the small profile tube trains share tracks with main line stock.
I lived a couple of stops north of Camden for a few years, and I quite liked having that flexibility. Much simpler just to wait a couple of minutes rather than dash through Camden Town station and try to figure out which platform the train would be on (iirc southbound trains for Charing Cross or the City could be on either platform, depending on which line it was coming south on; so you’d get midway through the station, look at screen with next departure and realise you had to go back to the platform you were on). Would still try to game the system, though: if it was more than a 5 minute wait for the train I wanted, I’d take the first one and see if I could save a couple of minutes by transferring.
According to TfL, there are plans to deal with the southern branching, though no mention of any changes at Camden: http://www.tfl.gov.uk/corporate/projectsandschemes/18091.aspx
And if this goes ahead, it actually will be a proper branching at Kennington rather than what’s effectively a T-junction: