An example of platforms designed for access from the concave side is at Lansdowne station in Boston, where side platforms for both the inbound and outbound directions are located to reduce platform gaps to commuter rail trains of the Framingham/Worcester Line. By contrast, a convex platform would leave the largest possible gaps between the car ends and the platform edge, making this design undesirable and thus rarely implemented. When passenger car doors are located only at the ends of each car (a common design for commuter rail and long-distance trains), platform access from a concave platform is preferred, since this brings the car ends in closest proximity to the platform edge. These types of gaps are geometrically intrinsic, and cannot be eliminated as long as the platform is located on a curved or banked segment of track or guideway. These spaces are caused by the geometric gap between a curve ( circular arc or otherwise) and the straight-line chord or tangent formed by a railcar or bus in proximity to a platform. Stations may have to use a compromise design, with a platform curved in a way that will allow a vehicle or train to arrive and depart without mechanical interference, but which leaves unavoidable horizontal and possibly vertical gaps between the cars and the platform edge. In real-world situations, stations are often constrained by limited space, legacy designs, and track geometry or roadway layout. Significant vertical and horizontal platform gap at University station on the MTR system in Hong Kong
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