Ask GGW: Why do some stations have side platforms?
Have you ever wondered why your Metro station has two side platforms instead of a single island platform? If so, you're not alone. Reader Sam Inman is curious, too.
Why are some stations (I'm particularly interested in the subterranean stations) designed with the side platform design instead of the island?Unfortunately, the answer is not simple. In practice, the layout can be influenced by a variety of factors, so there's great variation between transit systems. But there are some general rules that influence the layout (though they're not hard and fast).
Do you know if this is dictated by a topography/cost concerns? Or was there a design consideration that wanted to force passengers to make their decision at the mezzanine level rather than on the platform level?
In general, island platforms can be cheaper because they require less duplicative infrastructure. However, sometimes other technical factors can make side platform stations cheaper.
With an island platform, the station requires less vertical circulation. For example, Foggy Bottom only needs 1 mezzanine-to-platform elevator, since it has an island platform. But Farragut West needs 2 mezzanine-to-platform elevators, since it has side platforms. Staircases and escalators can do double duty at an island platform, but sometimes need to be duplicated at side platform stations.
Loading can also be an issue. For example, at a station that is very commuter heavy in the morning with passengers all traveling the same direction, a side platform station may have one platform that is very full and one that is completely empty. That's less efficient than an island platform, where the passengers can use the whole platform, even though they're primarily focused on one track.
But oftentimes these considerations take a back seat to the method of construction, which can also influence the station design.
For underground construction, if the line is cut-and-cover, it is often more cost-effective to build side platforms. With cut-and-cover construction, the tunnels are constructed by digging up the street, building the tunnels, and then rebuilding the street.
To build an island platform station with this method, it requires the two tracks be spread apart from each other (to give room for the platform). This requires more excavation than a side-platform station, which only requires the extra width for the length of the station.
So when subways are constructed using cut-and-cover, like along I Street around McPherson and Farragut Squares, stations often have side platforms.
On the other hand, when a subway line is deeper, and is bored through the ground, it often makes much more sense to have island platform stations. This is because when lines are bored, the two subway tubes are not directly next to each other. Since the tracks are already apart, when they get to a station, it's much easier to just put the platform between the tracks, rather than to pull the tracks together so the platforms can be on the outside.
This is the case for the deep stations along Connecticut Avenue and Wisconsin Avenue between Woodley Park and Medical Center.
It's much rarer, but sometimes both tracks are built in the same (larger) subway tube. This is the case for the Red Line between Farragut North and Woodley Park (not including either of those stations). Both tracks are in the same (bored) tunnel, and so at Dupont Circle, it makes much more sense to have side platforms, since the tracks are already right next to each other.
This is also the case for almost the entire Montreal Metro system, where the tracks are always in the same tunnel. As a result, almost every station has side platforms.
There's less pressure for one or the other design on elevated and surface rail lines, since the construction is cheaper than subway construction. However, there are still some influencing factors. For example, when the tracks are running in a freeway median, the road lanes have to spread out in advance of the station anyway, so there's no penalty for spreading the tracks out ahead of time either. So in that case, there's no penalty for an island platform station. For a side-platform station, the only penalty is the duplicated infrastructure.
On an elevated viaduct, it might be easier to have one structure carrying both tracks rather than two separate structures for each track, and therefore side platform stations may be cheaper, like at West Hyattsville. But then again, it's not necessarily better one way or the other, and so sometimes an island platform makes more sense, like at McLean.
Sometimes, though, a transit agency might make an intentional decision that overrides other concerns.
Terminal stations should have island platforms so that the next train can leave from either track. So any station that is planned to be a terminal for any period of time generally has an island platform. When a terminal does have side platforms, generally trains have to go out of service on one platform, go past the station, reverse, and then pull in on the other platform. That's very inefficient. Alternatively, passengers have to wait in the mezzanine and then pick a platform when the train is ready to depart, also inefficient. All of Metro's terminals have island platforms.
Any station that is likely to be a transfer between diverging lines should have an island platform. That way a passenger coming from one branch can transfer to the other branch simply by walking across the platform. This is the case at Stadium/Armory, where a passenger riding from Addison Road to New Carrollton can make an easy cross-platform transfer.
Furthermore, at key stations, certain platform arrangements can be more efficient.
Metro Center, Gallery Place, and L'Enfant Plaza are good examples. With right-side exits on the upper level, there can be multiple escalator shafts down to the island platform on the lower level. If both levels at these stations had center platforms, the only efficient layout would be to have a mezzanine between, which is how Fort Totten is laid out. And that's generally more expensive and less efficient. Though in the case of Fort Totten it works because the lines have such a great vertical separation (the Red is elevated, the Green is underground).
Other design factors
And while we don't have any examples of it in the Washington region, the "Spanish solution" can also be employed to reduce dwell times. The Spanish solution is where the doors on both sides of the train open. This makes it faster to unload and load the train. MARTA's Five Points station has this on both the upper and lower levels.
When looking at express/local configurations, having island platforms between the local and express tracks allow for an easy cross-platform transfer between trains going the same direction. But at some stations where express service needs to stop, but where the agency wants to discourage transferring passengers (because of crowding), the island platform can be placed between the two express tracks and with the local tracks having side platforms. This is the case at 34th Street/Penn Station on the 1-2-3 and A-C-E. That station is important enough that all trains need to stop, but the stations are crowded. The traditional island/island layout is present one stop north at Times Square/42nd Street to allow transfers between locals and expresses.
In systems that have express/local tracks, there are even alternate ways to accommodate local stations. In New York, local tracks tend to be on the outside, so local-only stations have side platforms. The drawback here is that if the local service ends before the express service, it's difficult to turn those trains around, since they have to cross over the express tracks. The Lexington Avenue (4-5-6) Line handles this by having the local 6 train dive under the 4-5 express tracks via the City Hall Loop.
A rarer alternate version is to put the local tracks in the center. In this case, with the express tracks on the outside, the local-only stations have center platforms. This is present on Chicago's north side trunk, with the local Red Line in the center and the Purple Line Express running on the outer tracks.
So, as you can see, the logic is somewhat complicated. Sometimes it's cheaper to do one, sometimes it's cheaper to do the other. Sometimes, there are logistical reasons for doing one over the other. Sometimes, it may just be more-or-less random.
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