What’s a Subway/El?
The rapid transit built in New York beginning with the first els codified two characteristics that spread to the rest of the US, and are often seen in other countries’ rapid transit networks as well. First, it is separate from surface transit – even when it did still have grade crossings, they were controlled railroad crossings, rather than street-running segments as is common on light rail. And second, it is separate from mainline rail.
Not much later than New York started building els, Berlin built the Stadtbahn, also an urban elevated railroad. However, it was meant to be used for mainline rail from the start, with two local passenger tracks and two long-distance passenger and freight tracks. Part of the impetus was to connect different railroad terminals within the city, which American cities did by building union stations disconnected from local traffic. Shortly later, Tokyo built its own mainline rapid transit system – the Yamanote Line bypass in 1885 and Tokyo Station connecting the Chuo and Tokaido lines in 1914. Both cities ran frequent local commuter service early, Berlin doing so even before electrification.
Of course, nowadays US regulations locked in the separation of rapid transit from commuter rail, but at the time, there was no such separation. New York could have built its subway to mainline specifications and run trains through to the LIRR. It didn’t because of historical accidents – it preferred compatibility with the els and even when the BRT chose a wider loading gauge for its own subway network, it still opted for narrower trains than on mainline track. At the time it seemed like no big deal, although some of the subway lines built were redundant with existing commuter lines (for example, the Flushing Line with the Port Washington Line). Again due to historical practice, commuter rail did not try to operate to rapid transit standards, keeping frequency low, and so nearly all urban stations closed. In both New York and Chicago, it’s often easy to figure out where the city ends or where the subway/L network ends because that’s the point beyond which commuter train stop spacing narrows, providing makeshift local service.
In subsequent decades, the German and Japanese approach proved itself much more capable of providing good transit to growing suburbs. In Tokyo, subways are legally railroads, and most lines are compatible with at least one commuter line in order to permit through-service. German cities have mainline rapid transit (S-Bahn) and also separate subways or subway-light rail combinations (both called U-Bahn). Many other cities and countries had to adopt the same system to increase transit ridership, at much higher cost since the necessary viaducts and tunnels connecting stub-end terminals were done much later. This is what led to the Paris RER, and what’s led to Thameslink and now Crossrail in London. Any other approach would require spending even more money on extending urban lines to the suburbs, exactly what’s done now in the two big suburban-focused US rapid transit systems, the Washington Metro and BART.
The kink is that despite the above problems of subways that are separate from both mainline and street rail, there’s now a different reason to build such lines after all: they can be made driverless. Most first-world cities already have legacy rapid transit or else have so much sprawl rapid transit is inappropriate, and third-world cities aren’t saving much money by eliminating drivers, but in the few cases of new builds (Vancouver, Dubai, Copenhagen, the newer lines in Singapore), driverless trains are common, and this allows trains to run more frequently, or even 24/7 in Copenhagen’s case.
This kink aside, there’s really no reason for a city to build a new New York-style subway, i.e. disconnected from light and commuter rail and running with a driver. Extending a legacy system is fine, but for new systems, there’s no point. This could be especially bad in growing third-world cities, which could find themselves paying too much for a subway they don’t need or unable to connect a subway they do need to the suburbs once they start suburbanizing. Third-world construction costs aren’t much if at all lower than first-world costs, but wages are much lower.
Some of the world’s largest cities have made or are making this mistake. Mumbai is building a new subway, on a different track gauge from the Indian mainline network, preventing through-service to the overburdened commuter trains. Shanghai and Beijing have vast subway networks, without express tracks or any ability for trains to run fast through city center; they have widely spaced stops so that they are faster than most other subway systems, but they have nothing on the rapid commuter trains in Tokyo. (Beijing is also developing a parallel commuter rail network, running diesel trains from the exurbs to the traditional city terminals at low frequency.) It works fine now, but when Shanghai grows and suburbanizes to the degree Tokyo has, it may find itself having to spend many billions on digging new tunnels.
Since a New York-style subway is inappropriate for new builds, some cities need to ask themselves which of the three kinds is the most appropriate. A subway-surface solution is mainly an option when one underground line can naturally split into multiple surface lines, as is the case in Boston, San Francisco, Cologne, and Frankfurt; this is because there’s a big difference between on-street and grade-separated capacity.
Tel Aviv, which is building a subway-surface line without any branching, is doing it wrong. For the other choice, I believe it’s a matter of how well-developed the suburban rail network is, and how much future suburbanization the city can realistically expect. In Tel Aviv specifically there’s also a separate element, which is that for religious reasons public transit does not run on weekend. If driverless technology makes the difference between trains that run 24/7 and trains that run 16/6, then it should be used even at the cost of otherwise worse service to some suburbs and destinations easily reached by legacy rail branches.
Finally, in North America, one of the reasons to engage in strong regulatory reform is to allow the mainline option to work. Some lines, for example the Harbor Subdivision between LAX and Union Station, should ideally host a mixture of local and rapid trains on the same tracks, and also allow intercity trains; if the Harbor Sub becomes an electrified commuter line then high-speed trains could serve the airport, providing a connection from the Central Valley to a major airport in addition to SFO, which would only get a station at Millbrae.
More in general, the only real disadvantage of legacy commuter networks is that they tend to not be very dense in the center of the city, requiring new builds; most of the Tokyo subway is just lines offering the commuter lines more capacity into the CBD, overlaying itself to also provide a tight in-city network. There’s no technical reason not to just build an electrified local mainline network as its transportation backbone, and if more capacity is required then build additional lines in the mold of Tokyo.
The elevateds were not originally separate from mainline rail. The els in Brooklyn had through-running from mainline LIRR; through service from the Rockaways to Chambers St BMT (using an incline from Atlantic Ave to the Broadway-Brooklyn el at Chestnut St) continued until 1917. I believe it was ended for regulatory reasons. There was also through-running from the els to surface lines, to reach Coney Island and other destinations; these were not all replaced by els until 1920. Even the Manhattan els had a short section of track shared with some New Haven RR trains 1891-1905.
Outside of New York there was even less of a dichotomy historically. In Chicago, surface interurbans ran through to the els until 1963. Boston, Philadelphia, and Newark had subway-surface lines, and Baltimore and Hoboken had streetcar els.
It seems to me that the strictly-separated rapid transit in the US is not particularly a result of the history of the oldest lines. I’d instead attribute it to the desire of freight rail operators (on the mainlines) and private motorists (on the surface) not to have to give up any priority to passenger transit, and regulators disposed to satisfy these desires.
The Brooklyn portions of the subway used to be excursion railroads, but as they were being shoehorned into the subway, there was sharp division between lines under BRT control that were connected and lines under other control (i.e. LIRR) that were not. And even beforehand, there was a dichotomy, leading to the building of an el on Fulton, a block away from the LIRR. Of course many of those els began their lives on the surface, but they were steam railroads rather than horse-drawn streetcars.
Elsewhere, yes, a lot of subways do descend from trolleys (Boston, Philadelphia, etc.), but there was separation between urban rapid transit and commuter rail. The companies that controlled urban transit were different from those that controlled mainline rail. For example, Boston had BERy, with some subway lines connecting to trolleys (today’s Green and Blue) and some connecting only to els (today’s Orange) or to nothing (today’s Red). It wasn’t just the rail operators and drivers who supported this division; city interests tried to prevent railroad moguls from controlling the urban transit systems.
In Cleveland, urban streetcarsand mainline intercity railroads were actually owned by the *same* people (the van Sweringens, for instance) for long periods. They *still* mostly kept them separate, so it’s interesting to ask why.
In the 1880s the Manhattan elevateds were owned by Jay Gould, who owned other railroads all over the country.
I wonder if there were fundamental problems with mixing steam engines and trolleypoles. Soot on the wires? It seems that very few lines mixed steam and overhead electric operation, particularly overhead electric; the Pennsy and Reading mainline electrifications are the first ones I can think of, and even there, for long periods certain tracks were known as the “steam tracks” and others as the “electric tracks”.
The safety problem of mixing giant, heavy steel vehicles (such as engines) with lightweight wooden passenger cars was recognized relatively early, but this applied to mainline railroads too, originally (until wooden cars were phased out entirely).
Editing correction: “It seems that very few lines mixed steam and overhead electric operation”. The redundancy is not needed….
For the most part it was because electrification was, with a few exceptions, due to legal or engineering factors prohibiting steam (such as tunnel ventilation), so there wouldn’t be much cause for using steam under wires. Separating the tracks on mainlines would be a common sense acknowledgement of their different performance characteristics I suspect.
Right, different performance characteristics. Of course. Steam has a weird acceleration profile. So basically there was a “mixed speed” problem.
The trains from Washington and Chicago didn’t change engines to go through Philadelphia. They changed engines at Manhattan Transfer a few miles out side of… Manhattan…
The trains to Exchange Place didn’t need to change engines and used steam until they were dieselized.
Manhattan Transfer was the engine change point because that’s where the electrification started, until some time in the 1930s when the wires reached all the way from Philadelphia to New York. Before then, Penn Station only had third rail and third rail DC locomotives were attached at Manhattan Transfer. Where, incidentally, H&M and PRR trains shared a track, if I’m remembering my track diagrams correctly.
The PRR started electrification with catenary in Philadelphia in 1915. The steam trains headed for Jersey City didn’t change engines. They blew smoke and steam all over the catenary. The steam trains going to Penn Station blew steam and smoke all over the catenary until they got to Harrison NJ where they changed engines.
The Reading began it’s electrification in 1931. The trains that ran past the electrification blew steam and smoke all over the catenary until dieselization was complete.
I believe it was ended for regulatory reasons.
The BRT didn’t need no stinking regulations. One of the reasons it’s the BMT and not the BRT.
Dual Contract stations start to open in Brooklyn around the same time. People start taking the El in much bigger numbers, some of them former LIRR passengers.
Newark had subway-surface lines
Newark still does. The one old line still running uses the former ROW of the Morris Canal. Downtown they covered it over and got a new street, Raymond Blvd. Outside of downtown they filled it in and it runs at grade. The extension to Bloomfield runs on the Silver Lake/West Orange Branch that used to be Erie and is still used by NS. Or is it CSX. One of them. The new shuttle between the Broad Street station and Penn Station runs underground part of the way, part of the way on the street.
Indeed; possibly the BRT had to stop allowing LIRR through-service or risk being regulated like a mainline?
Or the BRT made more money shoving people through it’s turnstiles than it did on the payments from the LIRR.
The LIRR ran trains at hourly off-peak frequencies. Why would it generate any nontrivial ridership?
What was the BRT running in 1908 when the first stations on what we now call the Nassau loop opened and what were they running in 1919 when the Dual Contract work was complete? When does what we now call Atlantic terminal open. And the subway stops at it’s door? The BRT needed the capacity and the it wasn’t really worth the effort for the LIRR, they could change at Flatbush and Atlantic to the subway…
The opening of the subway to Flatbush and Atlantic in 1908 was not enough for the LIRR to give up on through service, nor was the opening of Penn Station in 1910, but the opening of the westside subway in 1917 seems to have done it. At that point the LIRR could consolidate downtown and midtown passengers into a single terminal.
So what proportion of operating costs is train driver pay…?
I know it’s common wisdom that crew labor on trains is significant, but … it’s always seemed a little amazing to me. On a train carrying 1,000 people, it doesn’t seem like it would take too many tickets to pay the driver’s salary….
On very busy systems, the proportion is pretty low, but not negligible. Of New York City Transit’s roughly 46,000 employees, 3,600 are train drivers (3,000 more are conductors). The proportion of train drivers among subway employees is of course higher, since 46,000 counts both subway and bus employees; 11,000 of those 46,000 are bus drivers.
The issue is what happens in smaller cities. Vancouver and Copenhagen aren’t big enough to fill a train with a thousand people at any time, let alone at 11 at night. For large greenfield systems, driverless operation is just a nice thing to have, but for small ones, it lets trains run much more frequently at night than they could otherwise.
Stockholm, which is about the same size as Copenhagen, regularly fills subway trains with 1,000 people in the peak. So size isn’t the reason Copenhagen doesn’t. More likely it’s because Copenhagen also has the S-tog and because the Copenhagen Metro uses short, frequent trains (a deliberate design decision).
Think of it this way: A bus carrying 20 people has 50 times the driver labor cost per passenger, yet is still (marginally) viable.
In most Western European cities, buses are by far and large the major financial drain on any agency. This is the acute case of London (TfL).
I think this has to do, also, with the fact driverless operations can be planned in a whole different framework of increased frequency without the burden of additional manpower.
Let’s keep in mind that driverless operations also mean design features that further reduce costs, such as:
– complete isolation of line and platforms with PSDs (less security staff, much less disturbance from foreign objects on tracks etc)
– a necessarily more robust system to monitor trains (which can be used to reduce the need of manned dispatchers, supervision etc.)
– possibility of extended hours or even 24/7 service (creating and “available all time” value for costumers that usually they don’t find except in cars – occasional hourly twisting bus route doesn’t do the math)
– since the platforms must be gated, it’s easy to introduce some fare gate, further reducing staffing needs.
– the core group of employees that must keep working so that service is not disruptable in strikes is much smaller than a traditional operation. Keep this core group happy and properly compensated, and strikes are much harder to become disruptive.
SkyTrain in Vancouver has full automation without platform screen doors. Nor does the Detroit People Mover.
Nor does the DLR in London.
1. How expensive would it really be to make rapid transit compatible with mainland rail? For example, wouldn’t ripping out the BART tracks and putting in standard gauge be much cheaper than many of the projects now under discussion with Caltrain+HSR?
2. I believe the underground part of the Tel Aviv line is supposed to be much more frequent than the surface part. Not sure where the extra trains are supposed to go, maybe they will just turn around at the end of the tunnel?
I don’t understand why some people want BART re-gauged to standard gauge. Is it really worth the trouble? What would be gained by it?
2 million dollar subway cars instead of 5 million dollar subway cars.
What Adirondacker said. Using Indian broad gauge is weird enough that the premium charged by manufacturers for the cars is really large.
Also, to through-run BART/Caltrain trains down the peninsula.
1. My guess: very expensive, because of clearances. There’s no point in doing it for an existing system unless it’s almost compatible (the only example that comes to mind is the subsurface lines of the London Underground). Notably, Tokyo did not regauge the Ginza and Marunouchi Lines or replace their third rail with catenary.
2. Yeah, that puzzled me, too. They didn’t explicitly say anything about turnbacks. Close to the north end there’s a split to a depot while the trains are still underground, so trains could go there instead of continuing to the above-ground northern terminus, but there’s nothing like this at the south end. Judging by what little construction I’ve seen they’re going to build the train at-grade south of the tunnel. So I don’t know.
Regarding 2: The LRT peak headways are indeed supposed to be 90 seconds in the underground section and 180 seconds at-grade. I’m not entirely sure how they hope to achieve 90 seconds (40 TPH). Every second train will go to the depot at the east-end of the tunnel and turn-around. There is likely to be room for a switchback to the west of the cut-and-cover section just before the line turns south towards Jaffa.
Sorry, for ‘switchback’, read ‘siding’.
40 tph is possible as a limit case. I only know one non-automated subway that reaches this level, the Moscow Metro. 30 tph is a more common limit. Most likely Tel Aviv will have too much interference from surface trains – they’d go a bit off-schedule and then enter the subway late, delaying other trains.
Could tunnel diameter be an issue? For compatibility with mainline railroads, tunnels probably need to be somewhat larger in order to allow for overhead current gathering. This was done in Tokyo without extreme difficulty, but Tokyo uses 1500VDC. If electrified at the 25KVAC of modern mainline railroads, wouldn’t tunnels have to be larger still?
What about dual mode trains?
Slightly larger. The main issue for tunnel diameter, however, is not electrification, but “loading gauge”. That’s not standardized internationally. At all.
There’s a US mainline passenger standard (well, two, really, one for single-levels and one for bilevels); there’s a Continental European standard (“UIC”); there’s a British mainline standard, a London “subsurface” standard, a London “tube” standard; there’s a Russian mainline standard; there’s a US “old streetcar” standard (Boston / Philly / IRT / PATH); and there’s the US “newer subway” standard (BMT and several newer subways); and then there’s oddities like BART.
I think it is true that subways chose smaller loading gauges than mainline railroads in order to make smaller diameter tunnels.
Note that the London “tube” standard was *definitely* chosen in order to make smaller diameter tunnels than the existing “subsurface” standard; this is a matter of historical record. Nowadays the “tube” lines are considered uncomfortably narrow and all subway tunnels are built bigger than that.
I think that both the combination of mainline rail with rapid transit (i.e S-bahns) and the combination of mainline rail with streetcars (i.e. tram-trains) would be useful in the US. Both were once commonplace–your post focuses on the former, but the latter was readily common as well, and is now foreclosed by regulation (there are still quite a few places where FRA-regulated tracks can be found in the middle of streets).
TriMet managed to get away with running WES down the middle of SW Lombard Avenue in Beaverton (see here, and that’s an FRA-compliant service. The trains run mostly in the “median”, but the median in question is scarcely wider than the trains themselves. Since this is a spur that serves Beaverton Transit Center, no freights run down this track, but I suspect one conceivably could…
One thing to remember is that all these “hybrid” solutions involve some tradeoffs in terms of capacity and efficiency. Smaller cities like Karlsruhe can get away with it because they don’t need anywhere near the peak capacity of rail transit, but the bigger your city, the more capacity you need, and the harder it is to get away with these hybrid solutions. The Yamanote Line has its own entirely dedicated pair of tracks, and the Berlin Stadtbahn has two dedicated tracks for urban rail (the S-Bahn) with no connection to the two mainline tracks. Big city system with heavy traffic need that isolation so that they can run high frequency services reliably. But midsize cities only need that in the core of the system, while the outer branches have service only every 15 or 20 minutes and don’t need all the expense of full-on rapid transit. Berlin’s S-Bahn, for example, has single track sections and grade crossings on the outer parts of some branches, and there’s no reason BART could not have been built like that too.
there’s no reason BART could not have been built like that too
I suspect that the grade crossing issue is primarily due to ATO issues. In other words, BART wants the convenience and historical legacy of operating fully isolated network than to deal with the headaches that come with running at grade.
I don’t see any inherent tradeoffs (and the term “hybrid” seems weird, when the separation of “metro” from “non-metro” systems is largely artificial in the first place).
The idea seems very simple: when your “metro” hits the edge of the city, don’t stop, just keep goin’ (it doesn’t mean the line has to be exactly the same in all places).
The lines I normally take are interlined out the wazoo, and it works absolutely brilliantly.
It’s rather interesting that you wrote this up since I have a tendency to vasciliate in terms of my views on this. Experiencing German transit makes one appreciate the concept of a system in an average city that’s composed of a Stadtbahn for rapid transit, streetcar for local, yet heavily used bus routes, and a S-Bahn for longer distance intra-regional services, and RB/RE for what we’d consider the outer edges of the commuter rail network in the states. So sometimes, I’ll argue that WMATA* should have been mainline services with new tunnels on what ended up being the main alignments of the network, while the streetcars should have stayed or been upgraded to Stadtbahn principles. On the other hand, how does one move high ridership when you’re constrained with 2.8m width cars and and 50 meter length sets? Of course, there’s the Tokyo approach in which one argues for everything to built to mainline standards, but sometimes, you just want to cheap out, and not deal with the regulatory issues that are inherent in that.
*Or for other examples, that MBTA’s Blue Line should have stayed a trolley tunnel until modern light rail equipment was avaibale, while Philadelphia’s network should been a giant so-called “subway-surface” network instead of the two trunks formed by the MFL and BSS.
Japan builds subways to mainline standards to allow inter-running and reduce costs through standardisation, not to have wider trains. The JR loading gauge allows trains to be no more than 3.0 metres wide, and track gauge is only 1067mm because the builders of Japan’s first railway did indeed “cheap out”.
The Japanese approach to increasing capacity is to improve punctuality, increase frequency (for instance, 2 minutes 30 seconds during rush hour on the Yamanote Line) and extend platforms where possible to allow longer trains.
JR lines can run double decker trains, but they are not used on the busiest lines because boarding and disembarking take too long. As Alon has said, Japanese commuter lines are more or less indistinguishable from subways.
I’d actually argue that Mumbai is doing it right, and that it’s the *Indian government* which is doing it wrong. India started out with a mess of different rail gauges. The Indian government, for some inexplicable reason, wants to convert everything to Indian broad gauge. Mumbai, looking for standardized equipment with competitive bidding, built its subway in standard gauge.
The Indian government should be converting the mainlines to standard gauge. But it isn’t.
There are no standard gauge mainline railways in India. Most of the Indian railway network is 1676 mm broad gauge, so converting other lines to that gauge is the only practical option for standardisation.
As for the Mumbai metro not being broad gauge, loading gauge as well as track gauge matter for interoperability Some of the Delhi Metro lines are 1676mm, but mainline trains are 3660mm wide while Delhi Metro trains are 3250mm wide. Mainline trains are too wide for the metro tunnels and, if a Delhi Metro train ran on the mainline, there would be a 205mm gap each side between the train and the platform.
The thing is, India is basically assuming that it will never have international traffic, except to Pakistan (which is, uh, planning to switch to standard gauge for traffic to China and Iran). The assumption that international traffic won’t matter is a poor assumption, and seems to have been made unconsciously rather than deliberately.
When India started its conversion program, I believe it actually had more narrow gauge than 1676mm broad gauge. They chose the most expensive option *and* the one which is incompatible with China, Iran, and Thailand…. and with the former Soviet states, too.
Well, India is a vast country that can reasonably expect a large internal market by the time it gets rich.
There are advantages to broad gauge. One is that trains are more stable; because of both broad gauge and huge loading gauge, India can run double-stacked containers on regular flatcars, unlike the US and China.
Well, OK, there are some advantages. So…. put in sleepers for three-rail; Indian broad gauge and standard gauge can be built three-rail, unlike standard and Russian gauge.
If you do that, you at least have the opportunity to switch to standard gauge when you start needing international tracks. India is not even hedging its bets on its rail gauge.
Ok, so I was wrong. Until Project Unigauge 47% of the network was broad gauge and 45% was metre gauge. But your plan would still mean converting the entire network, which would be very slow, expensive and disruptive and would not provide any benefit for many years.
On the subject of international connections ,India has broad gauge connections to Pakistan and broad gauge and metre gauge connections to Bangladesh. To get to Thailand, which is metre gauge, you would have to go through Myanmar, which is also metre gauge. There is no connection between India and Myanmar, and none between Myanmar and Thailand.
If Pakistan is really planning to convert its entire network to standard gauge, it would be assuming that connections with India are not important. That might be true at the moment, but if Pakistan grants India most favoured nation trade status rail traffic from India will increase substantially.
Pakistan has been pretty disorganized lately and hasn’t invested in rail at all, but the most recent Prime Minister did say he intended to convert the network to standard gauge.
Political considerations mean that Pakistan is looking west to “Islamic” countries instead of east to its culturally similar neighbor India. Financial considerations also apply; China is really trying to figure out how to get a rail link into Pakistan and is willing to fund it if they can figure it out (it’s not at all straightforward to put rail through the one mountain pass between the two countries which isn’t in contested territory).
This is a complicated debate. Some countries have reasonably large networks of non-standard gauge. India (1676mm) and Brazil (1600mm) are examples. You also have the whole issue with Rusisan gauge, close enough to standard gauge that it doesn’t allow dual gauging without more expensive 4-railing.
Even within the Iberian peninsula this is problematic. Right now Portugal and Spain are having a rift about which direction to follow: The Spaniards built a high-speed network on standard gauge to integrate it with France and, in the future, Portugal. The Portuguese don’t want to complete mixed traffic high-speed line now because of the crisis, and the Spanish don’t want to upgrade Iberian gauge old lines to allow traffic from Portugal as they want to standardize high-performance lines.
However, when you have systems that are expected to have high frequency and operated as a closed system, going standard is much less of a problem.
How about some sort of through running between the NYC subway and LIRR/Metro North? EG: LIRR trains switching to the E, J, or Z tracks at Jamaica; MNRR Harlem and New Haven line trains switching to the B/D at Norwood/205th. The loading gauge, voltage, platform height, and current collection shoes are all different, but they are similar enough that you would think a technological solution would be possible. For example, build trains that can run at anywhere from 600-1000 volts; ‘kneeling’ trains that can raise or lower their suspension hydraulically by 3 inches to account for the 45 inch platforms of the B division and the 48 inch platforms of MNRR/LIRR; extendable ramps that can bridge the 4.5″ difference between difference between the 9’9″ wide B division trains and 10’6″ wide LIRR/MNRR equipment.
Would this be useful to anybody? The biggest obstacle other than that would be regulatory I suppose?
It was technically feasible 100 years ago and would still be today. But most of the subway routes that might be useful are at or near capacity, so using them to serve additional destinations beyond the edge of the city would not work well. Much more desirable is the opposite, using the in-city parts of commuter rail lines (which are mostly well below capacity, or at least their potential capacity if they were run competently) to serve more local destinations currently only served by overcrowded subways.
The only “underused” portions of NYC subway lines are some of the central express tracks on some of the outer lines. And obviously those couldn’t be used by anything with a wider loading gauge, as no extra clearance was left between them and the local tracks…. I suppose you could run BMT-sized trains out onto the LIRR if there were a reason to.
It depends on how you think of it. If you think of it as allowing commuter rail to run through to destinations it can’t now reach (think downtown), then it’s not too useful. As Anon256 says, there’s not much room on the subway lines to add commuter trains to. Plus it would be slow. It’s still faster to run into Penn Station and transfer there for a downtown train than to meander through Queens and Brooklyn even on a one seat ride.
But if you think of it as allowing existing subway frequencies to extend to currently unserved areas, there’s more merit in the idea. To be concrete, let’s look at Jamaica. If, somehow, the E, J and Z could from Jamaica Center find themselves on the LIRR tracks, then they could run through, one, say, on the Main Line, one on the Montauk Branch, one on the Atlantic Branch, to the City line. Since both LIRR and NYCT can’t use the same station (one uses PoP, the other faregates), decisions would have to be made on which stations get converted to NYCT use. Those could have their platforms lowered 3″. Perhaps previously abandoned stations could be restored. The benefit would be that a fairly large area of southeastern Queens would get subway service, at subway frequencies, 24/7.
There remains, of course, the (serious) problem of getting the trains from Jamaica Center onto the LIRR tracks and (serious) regulatory issues: not least that night subway trains would have to coexist with NY&A freights.
Whether the benefit exceeds the costs is a difficult question.
Or you could just increase the frequency of LIRR trains, saving all the construction cost and providing a faster ride to Manhattan without increasing crowding on the already-full subways.
Would this be useful to anybody?
I’m sure almost anything you could consider would be useful to someone but no it’s not particularly useful.