A platform gap (also known technically as the platform train interface or PTI in some countries) is the space between a train car (or other mass transit vehicle) and the edge of the station platform, often created by geometric constraints, historic legacies, or use of partially compatible equipment.
Many high-quality bus rapid transit (BRT) systems also use high platforms at station stops to allow fast and efficient level boarding and alighting, but potentially leaving hazardous gaps between the platforms and the buses. Alignment setups such as Kassel curbs help to reduce platform gaps without requiring time-consuming manual alignment at each BRT station stop.
Straight platforms
The ideal platform would be straight and align perfectly with a train or other large vehicle. Even in this case, a small gap between the conveyances and the platform is necessary to allow the vehicles to move freely without rubbing against the platform edge. In 2007, the Long Island Rail Road regarded an platform gap as typical on its non-curved platforms.
Curved platforms
In real-world situations, stations are often constrained by limited space, legacy designs, and track geometry or roadway layout. 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. 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. 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.
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. 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.
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. Concave and convex gaps also exist in several MTR stations in Hong Kong, particularly on the East Rail line, which was built on the historic Kowloon–Canton Railway line.
Gap fillers
Mechanical platform edge extensions known as platform gap fillers may be used to bridge the gap between platform and vehicle. These stopgaps require careful alignment of the vehicle upon arrival, and careful synchronization to avoid serious damage caused by departure of the vehicle before the extenders are fully retracted. They increase station dwell time, and introduce safety and maintenance concerns of their own.
Alternatively, the gap fillers may be mounted on the train, and linked to the door operating mechanism. They may be found on modern trainsets, like various versions of the Stadler GTW. Train-mounted gap fillers eliminate the need for careful alignment and, as the driver only gets the signal that the doors have closed when the fillers have fully retracted, require no special synchronization on departure. Moving all active components of the system to the train instead of the platform allows maintenance to be performed in a shop, rather than in the field. Singapore has committed to specifying its newer trains with gap fillers, to reduce the incidence of platform gap accidents in its crowded stations.
On the Berlin U-Bahn, which has two different loading gauges (Kleinprofil on U1-U4 and Großprofil on U5-U9), so-called Blumenbretter ("flower boards") bridging the platform gap were attached to Kleinprofil trains that ran on Großprofil lines at various times of rolling stock shortage.
Equipment compatibility
In some rail systems, significant platform gaps may also occur (both horizontally and vertically) because of equipment and platforms designed to different and somewhat incompatible height and width standards. This situation may occur especially when previously separate rail systems are consolidated, or start to interoperate, thus allowing equipment to be moved onto tracks where it had not been used before.