Subway Capacity: some remarks
March 24, 2014
…and the Vancouver Canada line case. The remarks apply also to LRT unless specified (another post has been dedicated to buses
In a nutshell, the person per hour per direction (pphpd) capacity a subway line can offer, is
- (capacity of a train) × (number of train per hour).
Like for buses, the capacity of a train is a function of different parameters, mainly person per square meter occupancy standard, and seats arrangement.
At the difference of low floor buses (and LRT), there is little “protuberance” (such wheel room) on high floor train, and technical room present in a train cabin rather under floor or on roof, are often the result of a tradeoff:
- train capacity vs easy maintenance
The theorical capacity of a train, is in fact a direct function of its surface:
- (length of the train) × (width of train).
…and a train length, is constrained by the station’s paltforms length, which are typically very expensive to expand.
below is an example of compared train capacity, expressed in term of surface able to accomodate passengers
|Train consist||Platform length||width||surface|
|Vancouver Canada Line||40||3||120|
|Vancouver Canada Line||50||3||150|
|Vancouver Skytrain (Expo line)||80||2.65||212|
|Paris typical subway line||75||2.37||178|
For matter of comparison, the theorical Canada line capacity (with 50meters platform) is just 15% lower than on most of the parisian subway lines, such as its line 2 or 5: those lines carry ~100million riders a year.
Behind the seating layout, a train needs in practice several features to effectively reach its theorical capacity. Among them
- Minimal unusable space between cars (and in cars)
- Allow passenger to “overflow” from a car to another one
Intercirculation between cars, usually allows that, but again, some interciruclation layout can be more efficient than other:
Dwelling time and frequency
homogeneous occupancy of a train is also function of the door disposition, but the door layout affect primarily the dwelling time. Short dwelling time is important for a host of reasons, frequency being one of them, and frequency affcet the line capacity:
- interval between train can’t be shorter than the station dwelling time
It is hence important to have as much as possible doors, but also have them wide enough, to allow good in/out flow movement. It is also important to avoid that some doors, slow down the boarding/alighting time because they have to handle more traffic flow:
- From a boarding viewpoint, where passengers have no apriori on the location of door on platform, the best way to do that, is to have all the doors equidistant (It make also the best use of the platform space)
- From an alighting perspective, all doors on a car should be equidistant
A single track, vs a double track, at the end of a line could be used as a cost saving measure, but obviously it affects the frequency of a train line. That said, if the single track portion is short enough, the impact can be relatively minimal.
- Frequency can be be obtained by using a tail track to store trains
The possible frequency is then:
- ((time to travel for and back the single track) + (dwelling time × number of train to be stored) ) / (number of train stored).
As an example, at Richmond Brighouse station, on the Vancouver’s Canada line
- the tail track past the station can accomodate one stored train , and the station another one
- the travel time between Lansdowne and Brighouse is ~90s and a typical station dwelling time ~20s
2 trains can run every 4mn on the Richmond Brighouse branch of the Canada line.
Because one train can run every 4mn on the Airport line, it is possible to get a train every 80s, or 45 trains per hour, on the common trunk (Bridgeport-Waterfront)
Even, with 40meters long train, the Canada line could provides a capacity of ~15,000pphpd, assuming 330 passengers per train: that is 3 times the actual capacity. Greater frequency are theorically possible with the introduction of short turn train (avoiding the single track section):
PS The above numbers for the Canada line, assume the availability of rolling stock, power supply, track signalling, and fast operating switch: All those could need to be upgraded, as well as the stations along the line to handle the corresponding increase in ridership, but it could be no need for heavy civil engineering work/track reconfigutation toward a capacity increase of 15,000+ pphd
 Addressing Canada Line capacity questions, Translink, June 3, 2010.