Rapid transit station design question

The scenario is a two track rapid transit line, could be subway or elevated.

What are the considerations that go into deciding whether a station has an island platform or two side platforms?

Thank you.

Passenger wrote:The scenario is a two track rapid transit line, could be subway or elevated.

What are the considerations that go into deciding whether a station has an island platform or two side platforms?

Thank you.

Here are a few:

* Cost (two side platforms requires twice the fare control equipment unless you locate it in a mezzanine level, another expense / canopies / concrete, etc.)
* Spatial considerations (island platforms usually require the tracks to 'bow' apart in order to accommodate the platform, which may not be possible, be more expensive, etc.)
* Operational/service considerations (island platform provides the ability to change directions without leaving the paid area or having to go up and over or down and under)

Thanks.

Any others?

Only other thoughts I can think of....

Crowd control - at a particularly busy station, crowding might make it desirable to use the larger area of two side platforms rather than an island platform.

OPTO (One Person Train Operation) - depending on the equipment cab layout, having many platforms on the opposite side of the cab from the operator's position can cause a longer dwell as he/she has to walk to the other side, open doors, then walk back after closing doors. I've noticed this on a system like the MBTA Orange Line (originally ran with conductors, now OPTO but many platforms on the left side away from operator) as well as WMATA (where it probably wasn't expected to be an issue with ATO, but is now that trains are largely run in manual mode). Eventually infrastructure can overcome this (closed circuit TV systems for platform viewing).

Most light rapid transit trains use center cabs, but I'll admit not all.

Dallas' DART makes a good study about using island or side platforms at stations.
(1) All elevated and below grade stations use an island platform. The costs for providing handicap access to two different side platforms is twice that for a single island platform.
(2) DART also likes to use an island platform at stations with pocket tracks for staging and turning trains (light rail trains having two cabs on each end). Pocket tracks aren't always located at the end of line stations, they can be found anywhere. Usually the pocket track is on just one side of the station, but there is one where the pocket tracks runs through a station with two island platforms.
(3) If the station is in the street (median or not) with auto traffic at the station, island platforms are used to place the trains between the pedestrians on the platform and the autos on the street.
(4) Almost everywhere else DART uses side platforms.
If there are exceptions to these 4 thumb rules, there aren't many.

Another consideration is a train to train transfer. With a center island, passengers from a NB train can easily switch to a SB train. Otherwise, some sort of stairway over or under the ROW needs to be provided. You could look at it as a way to get onto a branchline train from either direction. With a side platform, the passenger will have to wait for the next train to arrive.

In my day, the "green line or B" train went east to Jackson Park while the "red line or A" train turned west to 63rd/ Ashland. An AB stop was needed for passengers to swap between the A or B trains. A center island was needed to swap from a NB red train to a SB green train.

One location where the transfer gets complicated is when an elevated station is over the subway with a street in the middle. Passengers from one line need a free transfer to get from one line to the other without paying an additional fare. A modern new station may have an elevator built to connect the 2 stations.

PATCO (Philadelphia to Lindenwold) has island platforms throughout for the reasons mentioned by electricron above (all their stations are either elevated or in subway), and to facilitate door-closing, etc., in the absence of a full-width cab, the operator's position is to the left of center (leaving room for a railfan seat on the right).