Home View from Underground On the (obvious) development of subway systems

On the (obvious) development of subway systems

by Benjamin Kabak

Just like every subway in the world, New York has a core with branches.

In New York City, we like to think of our subway system as fairly unique. We have no true circle line, and our Outer Borough tendrils snake throughout the city. The amalgamation of old rights-of-ways and three independent builders, the current subway system arose through fate, fortune and good luck. Or so the story goes.

Two pieces making the rounds covering the same academic journal article beg to differ. As both Scientific American and Wired noted today, two scientists using two-dimensional spatial network analysis have determined general rules for any subway system. It’s interesting research, but it’s hardly groundbreaking to those who know urban planning.

Sarah Fecht of Scientific American summarized the “rules”:

First, subway networks can be divided into a core and branches, like a spider with many legs. The “core” typically sits beneath the city’s center, and its stations usually form a ring shape. The branches, which are more linear, extend outward from the core in many directions.

Second, the branches tend to be about twice as long as the width of the core. The wider the core, the longer the branches. And subway systems with more stations tend to have more branches. The number of branches corresponds roughly with the square root of the number of stations.

Last, an average of 20 percent of the stations in the core link two or more subway lines, allowing people to make transfers. [Physicist Marc] Barthelemy says his team does not know which factors are guiding subway networks to follow these general rules; perhaps the rules maximize efficiency. For example, too many branches or connections would be redundant and unnecessarily costly. In contrast, having too few branches would reduce the range of areas that the network services, and having too few connecting points would reduce travel efficiency.

Fecht notes, as an example, that New York City doesn’t have a clearly defined ring core, but even on that point, I beg to differ. It’s a four-train loop that isn’t particularly efficient, but the area constrained by the two IRT lines on the East and West Sides, the L along 14th St. and any of the 53rd, 59th or 42nd St. lines forms a loop around the city’s major work center. Lower Manhattan throws a wrench into this ring, but that’s what makes the city’s subway system works.

That’s neither here nor there though. New York’s subway system still fits under these rules as we have a clear core with branches feeding into that core. The branches are significantly longer than the core itself, and depending upon how you define the core, many of them offer multiple transfer points. Check, check, and check.

Of course, as interesting as this summary is, we’re not really learning anything new here. Subway systems exist to offer a cost-effective and relatively quick way to bring urban dwellers from mostly residential areas into and through the commercial centers. As the commercial centers generally form a core surrounded by other neighborhoods, a subway system will always follow this shape. That’s the whole point.

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al May 16, 2012 - 2:46 pm

The NYC subway system we have is the outgrowth of lines that covered a growing core that expanded from Downtown to Midtown. Much of the older infrastructure has been removed (the El in Manhattan) or replaced (the Loop downtown with the Chrystie St connections).

We also never built a true loop, but geography (the rivers constrain Manhattan and make expensive river crossings necessary), political boundaries (Hudson County isn’t NYC only because of the NY-NJ border), and zoning (density restriction in Brooklyn and Queens) play important roles in that.

al May 16, 2012 - 2:56 pm

PS the PATH should be considered as part of the system. When combined with the A,C,E,G,M lines, it forms a loop of sorts around and through the CBD.

Jeff May 17, 2012 - 9:13 am

The Nassau Street loop (and the unbuilt Centre Street loop) was supposed to be our versions of the typical CBD loop. Its just that NYC’s main CBD has shifted from Lower Manhattan/Downtown Brooklyn to Midtown since the early days of the subway system.

Jason May 17, 2012 - 3:25 pm

What was the Centre Street loop?

John-2 May 17, 2012 - 9:55 pm

The original plan before the Nassau St. connection was built was to loop the line back across the East River via the Brooklyn Bridge.

John-2 May 16, 2012 - 2:46 pm

New York’s development differs in that the city had a dual core initial framework out of the 19th Century that involved shared downtowns in Manhattan and Brooklyn, along with the Midtown core which developed with the initial subway lines in the first part of the 20th Century.

That explains the Nassau Loop and the incredible density of lines between Borough Hall and the Atlantic Avenue-Flatbush Avenue junction in downtown Brooklyn, which is as dense with rail lines as any area of Manhattan. And that’s true in part because the IND added to the mix even after the current city business patterns were well into their current development by the late 1920s — not so much because the area needed more rail lines, but because the city wanted to kneecap the BMT’s elevated lines through downtown Brooklyn.

The same thing is true of at least the Sixth Avenue IND in Manhattan. You can’t look at the city’s system as it stands without taking into account that almost a third of it wasn’t built to expand or develop new core areas, but to usurp service within the existing cores from the existing operators. The IRT and BMT’s outer reaches grew more organically as laid out in the Scientific American article; outside of the Queens Blvd. line, there wasn’t much organic about the IND’s line development, especially if you overlay the now-defunct el lines that were already servicing much of it’s coverage areas.

SEAN May 16, 2012 - 2:57 pm

Better examples of the core & spoke are the WMATA & the CTA. For the MTA the core extends from the L along 14th Street to the F along 63rd Street south to north & 8th Avenue to 2nd Avenue west to east. However there other areas in the city that can qualify as cores as well. These include 74th Street Jackson Heights & Atlantic Avenue station discussed yesterday.

ant6n May 16, 2012 - 5:17 pm

The New York subway system is a complete mess; and badly planned (because it wasn’t). In practice it doesn’t matter all that much, because the development of the city in the last 100 years sort of followed the subway, and made things fit together a bit better.

UESider May 16, 2012 - 9:09 pm

this is hardly insightful – I hope it didnt take a PhD to write these rules!

it sounds more like a description of the London and Chicago transit systems as well as almost every highway near a city, such as Baltimore, DC, Charlotte, Boston (somewhat), and plenty of others

i really dont see the use for this, but thanks for posting

Alon Levy May 16, 2012 - 9:52 pm

But they have charts!

No, seriously, you’re right. The quantitative parts of the paper are the ones with the exceptions for geography, planning, etc. The qualitative core-branch question is universal, but it’s a pretty obvious corollary of the high-cost, high-capacity nature of rapid transit.


Ed May 17, 2012 - 12:22 am

This is one of those thoughtful pieces on this blog that I completely disagree with, to the point of wondering what Ben is smoking.

The New York subway system is a classic case of quantity over quality. It was built by two private companies whose lines were connected in a jerry-rigged manner later on, supplemented by “independent” municipally owned lines designed primarily to bankrupt the private companies. It mainly used elevated lines (actually so poorly constructed that its impossible for the MTA to build new elevated lines now, even though the tech now is reasonably clean and quiet) in what at the time was the most densely populated system in the world.

The subway system in New York really works as well as it does because of the city’s size, wealth, and geography. The city was large enough and wealthy enough that even if the city government couldn’t be bothered to put in a proper transit system, businessmen would do that, and persevere in the hope of profit even in the face of every government effort to put them out of business. Because the city is a archipelago, the routes are pretty obvious so its impossible for even the plans for two competing private lines to screw things up much. And because the city has always had high density and has been built on islands, there was no question of just tearing down the system and relying completely on motorized transport during the post World War II cheap oil era.

I appreciate the end result, but one of the difficulties of making recommendations for the future is that, for good or ill, “planning” really didn’t get us here.

Jeff May 17, 2012 - 9:20 am

Agreed that the NYC subway is quantity over quality. Too many redundant stations and routes that over-serve certain neighborhoods (such as Lower Manhattan/Downtown Brooklyn) while other densely populated neighborhoods don’t get enough. Too many lines on the west side of Manhattan and only one line on the east.

Part of the reason is due to the age of the system and the demographic shift happening through the years. Elevated trains that were supposed to serve some neighborhoods get torn down without being replaced, etc., not to mention the fact that the system was designed to cannibalize itself.

Alon Levy May 17, 2012 - 3:09 pm

The overserving of Lower Manhattan and Downtown Brooklyn is expected: those were the two biggest CBDs in the city in the early 1900s.

What’s less excusable is the underserving of Queens. In the 1900s and 10s it was barely populated, but by the time they were building the IND it was already on a par with the Bronx, growing about as fast, and with much more potential for further growth. Instead the borough got too much expressway infrastructure and 1 or 2 subway tunnel pairs to Manhattan too few.

Andy Battaglia May 19, 2012 - 5:06 am

During rush hours I would hardly call any station in Manhattan “over-served.” Even all the Wall Street stations are crowded. If all those small stations were combined into 6 track mega-stations, no one would be calling them redundant in the first place. I suppose one could call Downtown over-served only in the sense that it isn’t dangerously over-capacity like the midtown stations along 53rd street are. I for one am grateful that downtown has a little breathing room.

Think twice May 17, 2012 - 10:49 pm

“It was built by two private companies whose lines were connected in a jerry-rigged manner later on, supplemented by “independent” municipally owned lines designed primarily to bankrupt the private companies.”

To be specific, the private companies were merely competing for routes that were already laid out by the city’s own committees.

Tsuyoshi May 17, 2012 - 9:27 am

What actually makes the New York system unique (and extremely confusing to visitors) is one thing not mentioned here: express trains. Of course express trains exist in other cities, but they are, with a few exceptions, confined to “commuter rail”.

The Cobalt Devil May 17, 2012 - 9:44 am

True, the tourists don’t seem to be able to distinguish between express and local, but imagine where NYC would be without express trains! One of the few things actually DESIGNED into the system that works well and other cities (surprisingly) don’t copy much.

Steve May 17, 2012 - 4:11 pm

Except on fairly long trips, the time benefit from expresses is actually pretty small relative to wait times. That said, the capacity and reliability benefits from having intersecting but distinguishable systems is huge. Imagine how much bunching there’d be on the Lexington Ave line if each train stopped at each UES station.

But most other cities don’t have New York’s sustained density and so don’t need the capacity afforded by expresses — building another line serving another neighborhood is a better investment for a city built like DC, London, etc.

Alon Levy May 17, 2012 - 10:52 pm

It’s not really a matter of capacity. Tokyo and Seoul, whose rail lines are mostly two-track, and Shanghai, whose rail lines are all two-track, have worse capacity issues than New York.

Rather, it’s a matter of construction techniques. New York is unique in that,

1. It built its subways early, by hand,
2. Its subway is subsurface rather than deep-level, so that construction was cut-and-cover, and
3. It has wide enough streets for four tracks.

Notably, the parts of the system that were not subsurface, i.e. the underwater tunnels, were all two-track. When it costs almost the same to build four tracks as it does two, of course cities will build four tracks. But when it costs twice as much, as is the case today with TBMs and with any deep-level construction, cities will build two tracks, and if there’s money they’ll build two two-track lines.

Steve May 18, 2012 - 12:04 pm

Fair enough. And you’re right that I should have thought about Asian cities as places with New York’s density.

But I also think there are plenty of examples of 2-track cut-and-cover lines. Aren’t London’s subsurface lines 2 tracks? Wasn’t the older part of Boston’s Red Line cut-and-cover?

Alon Levy May 18, 2012 - 5:24 pm

In London the streets are narrower.

In Cambridge, I’m honestly not sure. Could be that BERy didn’t think it needed four-track capacity? Or maybe it had always intended to continue under Summer Street, which is too narrow for four tracks, again…

Miles Bader May 18, 2012 - 6:00 pm

Though you can have express trains on “two track” systems too, by using passing tracks in stations…

Alex B. May 17, 2012 - 10:26 am


You write: The amalgamation of old rights-of-ways and three independent builders, the current subway system arose through fate, fortune and good luck. Or so the story goes.
Two pieces making the rounds covering the same academic journal article beg to differ.

But they don’t really beg to differ. That’s not what the paper is saying. The summary is saying, instead, that despite of all of the independent and rational decisions (both in New York or in DC or London or wherever), the end result is remarkably similar across the world, despite those local quirks of ‘fate, fortune, and good luck.’


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