IBX Cannot be a Tram
Clay Guse of the NY Daily News reports that in New York, the plans for the Interborough Express connector between Brooklyn and Queens are starting to lean in the direction of light rail. To be very clear, light rail in this context just means running light rail vehicles on infrastructure that is entirely grade-separated, either in the Bay Ridge Branch right-of-way (which has a handful of freight trains and is mostly wide enough for light rail and freight on separate tracks) or on viaducts (over the sections of the branch that are too narrow). I do not think there is any plan to downgrade IBX to a tramway. However, on Twitter I was asked about this anyway: why not make it a tramway, for more on-street flexibility?
What is a tram? Or a streetcar? Or light rail?
A tram or streetcar is a rail vehicle that runs predominantly on-street. The quality of the right-of-way may vary, from full mixed traffic as was traditional, to dedicated lanes that may be shared with buses and emergency traffic, to a grassy median that is no longer usable by road vehicles. But the distinguishing feature of the streetcar is that it runs on a street.
The doesn’t mean the streetcar has to run on-street the entire way. Street running is slow, even with dedicated lanes. Paris’s T3, an orbital tram in the grassy median of the Boulevards des Maréchaux on the outer margin of the city, averages 18 km/h. Berlin’s streetcars average 19 km/h; a handful of central sections are mixed-traffic but most have dedicated lanes, and in outer parts of the city there’s just less traffic and lines are generally faster.
There are two main ways to speed up the streetcar: make it faster in city center via tunneling (called subway-surface, Stadtbahn, or premetro), or make it faster outside city center by finding grade-separated rights-of-way (called tram-trains). Confusing, both subway-surface and tram-train systems are called light rail in the US, and Germany’s most celebrated tram-train, that of Karlsruhe, is also called Stadtbahn. Because these systems have evolved from all-surface streetcars, the separation between them and streetcars is not always perfect, which is why the American distinction between light rail (either subway-surface or tram-train) and streetcar (all on-street) is sometimes muddied in popular reporting.
Can IBX function as a tram variant?
No.
The problem with running an orbital tram parallel to the right-of-way is that there is no good street for it to run on. On the map below, the thick black line denotes the right-of-way that IBX is to use:

There are no on-street alternatives to the right-of-way. Brooklyn has three major orbital buses: the B35 on Church, and the B6 and B82. Church is not wide – dedicated lanes there would be contentious and still produce inferior speeds to those of T3, let alone streetcars in less dense cities; it’s a great corridor for dedicated bus lanes, but not for a tram. The B6 and B82 shift between different streets, as do other crosstown routes, like the B1, B3, B8, B9, and B11. Even Kings Highway is only 24 meters wide.
This, in turn, is why IBX is such a great idea: it provides service that the surface bus networks can’t provide, because the quality of rights-of-way is poor unless one uses the Bay Ridge Branch. When the street network is poor, surface transit ridership is suppressed relative to travel demand, which means that a rapid transit service like IBX will overperform any model trained on existing travel volumes.
This is also why no variant with any street running is viable. Not only is there no good street for a streetcar, but also there is no section of a street that is good for a streetcar. The narrow sections of the Bay Ridge Branch right-of-way, mainly the segment between the F and Q trains, don’t parallel any convenient street.
Moreover, subway-surface alignments work by branching the grade-separated core into many surface branches, but there is no good tie-in. Circumferential lines sometimes do branch, but the best use case is when there are major destinations just off the route. This is not the case for IBX: Brooklyn College is on-route. The most significant destination in Brooklyn off the route is Kings County Hospital/SUNY Downstate, which is unusually poorly-served by the street network even by Brooklyn standards, and is therefore only on one bus route, the B12, rather than at the intersection of multiple buses as it ideally should be. There is no viable surface deviation off of the IBX right-of-way that serves it.
So why light rail?
The modal alternatives analysis seems biased in favor of light rail. This, to be clear, is not light rail as a service or infrastructure technology – the plan is to use viaducts wherever the Bay Ridge Branch right-of-way is too narrow for IBX and freight tracks side by side. Rather, the plan is to use light rail vehicles on a service that is entirely rapid transit.
This has precedent in the United States. In the same manner that historic streetcars evolved into subway-surface lines in Boston, Philadelphia, and San Francisco, and into the tram-trains that are called light rail elsewhere (with inspiration from Germany, brought in by American troops serving there in the Cold War), some light rail lines evolve into fully grade-separated rapid transit. It’s uncommon, because usually the parts that are left on the surface are the most difficult to construct, but it does exist. The Green Line in Los Angeles runs LRVs on a fully grade-separated right-of-way, mostly in the median of the 105, and the Gold Line’s initial section to and beyond Pasadena has just 1.5 km of street running, on Marmion Way. In Calgary and Dallas there are plans to bury light rail lines, which could result in fully grade-separated lines that still run LRVs and are locally conceived of as light rail.
But in New York, this is not a wise course of action. Running rapid transit with LRVs is great for a city that has LRVs but not subway trains, like the Los Angeles of the early 1990s. A city with both may potentially still elect to use LRVs if it expects some surface extensions. But New York has large-scale operations and maintenance for subway rolling stock, and none for LRVs. The only light rail in the region is in Jersey City and Newark, which do not share management or maintenance facilities with the city, and couldn’t do the latter even if they wanted to since they’re on the wrong side of the Hudson.
If intermediate-capacity transit is desired, New York could build shorter platforms, only long enough for 4- or 5-car trains. If even less capacity is desired, it could go down to 2-car platforms; the rolling stock would need to be somewhat captive to the line, since the rest of the system runs permanently coupled 4- and 5-car trains, but that’s completely normal for a large subway system, and heavy maintenance facilities can still be shared. I’m wary of reductions in capacity just for the sake of downsizing – this is an entirely above-ground project, so station costs are not as onerous as they are underground – but I can see a case for smaller trains.
I can’t find a good reason for this preference for light rail over subway equipment for what is, by infrastructure and service, rapid transit. I can find many bad ones, of which the most likely is a desire for something different from the subway with all the connotations it has.
But this does not mean that the IBX plan is a tram. It’s not; it’s rapid transit service, which could easily be a normal subway, running LRVs for bad reasons.