THE DEVELOPMENT OF URBAN MASS transit systems has exploded in recent decades as the world’s population has rapidly urbanised. In recent years, mass transit and subway systems around the globe have carried approximately 55 billion passengers per annum. Since the turn of the millennium, thousands of kilometres of new rapid mass transit and subway systems have opened in cities globally. And numerous light rail and metro systems and expansions are currently under construction or in development.
While these modern rail system expansions are operationally similar, they each have different design characteristics, such as high voltage AC or DC traction power, various mechanical ventilation systems, energy-optimized propulsion and regenerative braking, light rail vs. heavy-rail rolling stock, and different operational standards. However, one of the most fundamental differences in the future, will be the transition to fully-automatic (driverless) operation.
Automatic train control (ATC) will be an important feature in allowing rail systems to convey larger ridership capacities in the future, but this shift away from the conventional human train operator adds new safety and operational risks that must be addressed. In order to maintain passenger safety, improve air quality and support the future operational needs of their systems, transit agencies must also consider strategic measures such as platform-edge doors (PEDs) or platform screen doors (PSDs).
The numerous benefits offered by PEDs mean that transport agencies are increasingly adopting their use both in new mass transit and metro designs, as well as in the retrofitting and refurbishment of existing lines (as currently seen in London, Paris, Barcelona, Hong Kong, Singapore, Chennai, Vienna, Hamburg, Doha, Taipei, Shanghai and Beijing, to name but a few). Additionally, there are several agencies exploring PED implementation with pilot studies and test installations.
ESSENTIAL FACTORS FOR PED INSTALLATION
This article considers the following essential factors for PED implementation:
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By GlobalData- The integration of PEDs into rail operation concepts, including emergency responses.
- The impact of PEDs on the tunnel climate, which is separated from the station climate when using full-height PEDs (consequently, the station/ platform climate also needs to be addressed).
- Tunnel ventilation performance which requires specific modelling of the underground systems, including the PEDs for both normal and emergency operation.
- Planning aspects (e.g. space required for the doors, location of a control room at track level, earthing, remote opening, emergency opening, air leakage in the PED arrangements and its impact on air quality, evacuation, etc.).
- Aerodynamic loads on PEDs related to train movement.
Recent international tunnel ventilation systems are normally associated with PEDs that completely enclose the platform area, separating it from the trackway. However, detailed modelling and simulation must be undertaken during system integration that considers all potential operating modes of the ventilation system to determine its effect on (and interaction with) the PED system and how they combine with the aerodynamic effects of moving trains. In particular, pressures generated by the operation of ventilation systems for smoke extraction or fire management should not inhibit the emergency operation of doors.
The aerodynamic loads (pressure transients) arising from the movement of trains past PEDs that completely enclose the platform area must also be considered in the structural design of the barrier, as well as the supporting structures. In addition, the fatigue effects of aerodynamic cyclical loading from passing trains should be considered for all platform barrier systems.
Full-height PEDs at sub-surface or enclosed stations may be used as a direct fire barrier, to minimize the spread of flame and/ or smoke in the event of a fire (on either side of the barrier) or simply to segregate ventilation in the station and track areas. However, if a platform barrier system is intended to be used in this manner, it should conform to all additional and applicable fire standards.
HALF-HEIGHT PEDS
There are basically two different types of platform edge doors, half-height and full-height. From experience, half-height PEDs are typically used in the following cases:
Refurbishment of existing stations
When existing stations are retrofitted with PEDs, half-height doors offer a number of advantages. They include patron safety; lower capital and maintenance costs compared to full-height doors; the continued use of existing ventilation equipment (fan dimensions, volume flow rates, etc.) with no further retrofitting required; and no conceptual changes to operational rules and emergency scenarios.
Stations with high ceilings
In stations with high ceilings, it can be advantageous to use half-height PEDs to compliment the design/architecture of the station and to minimise capital and maintenance costs. However, the use of half-height platform doors in a station must be primarily determined, among other things, by the safety and fire protection concept.
Above-ground installation
Even above-ground, PEDs can be useful in protecting passengers waiting on the platform from the aerodynamic forces induced by the rail operation (as well as other safety considerations, such as track incursions and platform overcrowding).
FULL-HEIGHT PEDS
New stations
For new underground stations, it is much easier to integrate full-height PEDs than for existing stations. The reasons for this are that PEDs can be considered architecturally at an early stage and the ventilation and smoke extraction concepts can also be adapted accordingly.
Special climate zones: If a station (above or below ground) is located in a demanding climate (warm or cold), full-height PEDs can provide passengers with higher levels of thermal comfort on the platform and adjacent mezzanine levels.
Station and emergency ventilation system refurbishment: Projects involving the system-wide or significant refurbishment of existing stations, station ventilation and tunnel ventilation systems, offer the opportunity to consider installation of fullheight PEDs as the added capital cost may be offset by other system efficiencies that are gained as a result. Here, engineering and lifecycle cost assessments can help to determine the potential benefits.
JUSTIFICATION FOR PLATFORM-EDGE DOORS (PEDs)
Disruptions on train platforms are all too common and frequently result from objects falling onto the track, such as wheeled suitcases, bags, telephones, and the like. This can lead to several consequences:
- Objects can damage train components (e.g. hydraulic lines, cables and sensors). Where larger objects such as suitcases are concerned, track cleaning can also lead to disruptions in operational continuity.
- Objects can initiate a fire if, for example, a newspaper or similar waste is caught up in the undercarriage and heated by the braking system or gets trapped between the third rail and the current collector.
- When personal belongings fall onto the track it is likely to encourage owners to enter the trackway to retrieve it. This endangers lives and disrupts operations.
The most tragic situations are where people’s lives are lost. There are situations where people aim to commit suicide, but there are also unintentional falls, acts of violence and platform overcrowding that cause people to fall onto the tracks. In addition to the sad loss of life, the lives of transit employees and witnesses are also very negatively impacted by such incidents.
INFILTRATION / EXFILTRATION, AIR QUALITY AND CLIMATE
Closed, full-height PEDs separate tunnels from platforms aerodynamically. Any air entering the platform from the tunnel (infiltration) or from the platform to the tunnel (exfiltration) either passes through leakage areas of closed PEDs or through open PEDs when a train stops at the platform.
Major exfiltration and infiltration of tunnel air to the platform occurs through open PEDs due to the piston effect of trains moving within the tunnel network. Exfiltration and infiltration air-flow rates are variable and depend on various parameters, such as:
- The number of trains that are running in the system
- Train sections relative to the tunnel section (blockage ratio)
- Train length and speed
- Dwell time in stations
- The resistance of the draught relief path.
Typically, for high-quality PED systems, only a minor contribution to exfiltration and infiltration occurs through leakage of closed PEDs.
Residual exfiltration and infiltration rates have significant impact on:
- The volume flow-rates of the station platform ventilation and air conditioning systems and hence, on the sizing of the equipment (fans, ducts, air unit handler), the energy consumption and the thermal comfort on platforms.
- The air quality on the platform and in the station, as lower infiltration rates reduce the quantity of rail dust and polluted air entering the station.
- The additional maintenance costs that are associated with the replacement of filters, and the cleaning and repair of equipment in the station’s back-of-house and public areas.
EMERGENCY TUNNEL VENTILATION EFFICIENCY
In the case of a fire in a tunnel section adjacent to a station, the operation of tunnel ventilation fans (in the case of no PEDs or half-height PEDs) results in part of the ventilation air escaping through the adjacent station openings (supply air) or air being drawn in by fans (exhaust air).
This reduces the overall ventilation system efficiency because only part of the air volume is delivered to the fire’s location for ventilating the incident. In contrast, full-height PEDs can improve emergency ventilation system efficiency, by reducing air leakage at stations, allowing the maximum air volume to reach the incident. As a result, tunnel ventilation can be operated in a more targeted manner, and the emergency fan airflow rates can be reduced, or the tunnel ventilation concepts can even be simplified when applying longitudinal ventilation.
TRAIN OPERATIONS
The primary safety function of PEDs is the control of public access to the tracks during periods when a train is not located at the platform. PEDs provide an active barrier allowing greater passenger densities on the platform, which is particularly important during rush-hour services and when special events such as concerts or sporting events occur. The use of PEDs allows greater flexibility to effectively optimize transit services to include skip-stop services or express routes, reducing travel times and increasing system capacity. An additional advantage of using PEDs is that operational safety is maintained without the presence of a train operator.
CONCLUSIONS
The adoption of PEDs should be considered at an early stage of a project and requires close coordination with vehicle operations (train control systems), fire protection (smoke extraction at platforms, tunnel ventilation), and emergency egress concepts, as well as the station architectural concept and design. PEDs allow transit system operators to more effectively manage both passenger safety and environmental conditions on platforms and stations. In particular, air quality management is a topic of current investigation by many transit agencies and post-Covid ventilation mitigations are important to both public perception and transit agency policy.
Half-height PEDs offer several safety advantages, whereas full-height PEDs offer additional environmental control advantages in both, normal and emergency modes. Detailed tunnel ventilation assessments are required to identify potential benefits.
Growing urban density and increasing ridership exert operational pressures on existing transit systems for reductions in headway. However, reductions in operating headways have a minimum interval beyond which existing train control systems are able to provide industry-accepted levels of safety. As a result, existing operators will find that system-wide upgrades to automatic train controls (ATC) will need to be undertaken in the near to medium-term future.
