Platform Screen Doors (PSD): Types, How They Work & Benefits

Platform screen doors explained: full-height vs half-height PSD, how they work with CBTC and ATO, safety benefits, energy savings and which metro systems use them.

June 11, 2026 8:27 am

Quick Answer — Platform Screen Doors

Platform Screen Doors (PSDs) are barriers installed at the edge of railway or metro station platforms that open and close in synchronisation with the doors of a stopped train. They physically separate the platform environment from the track, preventing passengers from falling or stepping onto the live rail, blocking objects from the track area, and — in underground stations — enabling platform air conditioning by creating a sealed environment. PSDs come in two main types: full-height (floor-to-ceiling, completely sealed) and half-height (waist-to-shoulder height, not sealed). They are a prerequisite for driverless (GoA4) metro operations and are increasingly being retrofitted onto existing systems worldwide to improve safety and capacity.

What is a Platform Screen Door?

A platform screen door — also called a platform edge door (PED) in its half-height form — is a sliding door panel installed along the platform edge of a railway station, aligned with the doors of the train that serves that platform. When a train arrives and its doors align with the PSD panels, both sets of doors open together; when the train is ready to depart, both sets close together before the train moves off.

The concept was first implemented at scale in 1987 on the Singapore MRT, which opened as one of the world’s first fully automated heavy metro systems with full-height PSDs from day one. Since then, PSDs have become standard on new metro systems worldwide and are progressively being retrofitted onto older networks.

PSDs are distinct from simple platform edge barriers or yellow safety lines painted on platforms — those are passive safety measures, while PSDs are active systems that physically block track access and are integrated with the train control system.

Types of Platform Screen Doors

Type	Description	Key Benefit	Typical Use
Full-Height PSD	Floor-to-ceiling glass or steel panels, completely sealing platform from track. Fixed frame with sliding door panels aligned to train doors	Complete environment separation — enables platform HVAC, maximum safety, full noise isolation	Underground metros, driverless systems (GoA3/4), hot or cold climates
Half-Height PSD (PED)	Waist-to-shoulder height barriers — typically 1.2–1.5 m tall. Does not seal the environment but creates a physical barrier to the track	Lower cost, easier retrofit, still prevents falls and intrusions — suitable where full sealing is not required	Elevated or open-air stations, retrofits on existing lines, light rail
Automatic Platform Gates (APG)	Similar to half-height PED but with swing or retractable gate sections rather than sliding panels. More flexible configuration	Adaptable to varying door positions across different train types operating on the same line	Mixed-fleet lines where train door positions vary
Movable Platform Barriers (MPB)	Full-height but with individually addressable door modules that can be repositioned to match different train door configurations	Future-proofs the station for different rolling stock generations without structural changes	New stations designed to accept multiple train types over their lifetime

How Platform Screen Doors Work: The Technology

Door Alignment and Precise Stopping

The most fundamental technical requirement for PSDs is precise train stopping. The train’s doors must align exactly with the PSD panels — typically within ±50 cm for half-height systems and ±30 cm for full-height systems. On older signalling systems, this precision was difficult to guarantee consistently, which is one reason PSDs were rarely used before modern train control systems became widespread.

Modern CBTC (Communications-Based Train Control) systems provide continuous position data accurate to within a few centimetres, making precise stopping routine. On CBTC-equipped lines, the automatic stopping system brings the train to rest within the required tolerance at every station on every stop. This relationship between CBTC and PSDs is why new metro systems almost universally adopt both technologies together.

Door Interlock System

PSDs are integrated into the train’s door control system through an interlock circuit. The sequence at every station stop is:

Train arrives and stops within the required tolerance window
Train door control system sends an “at station, doors release” signal to the PSD controller
PSD controller confirms train position and unlocks PSD panels
Train doors and PSD panels open simultaneously (or PSDs open fractionally first)
Passengers board and alight during the dwell period
Train door close command issued — PSD panels and train doors close together
PSD controller confirms all panels closed and locked — sends “platform clear” signal to train
Train receives departure clearance from the signalling system — departs

If any PSD panel fails to close and lock, the train cannot depart — the interlock prevents departure with an open gap. This fail-safe design means a PSD fault causes a delay rather than a safety incident.

Obstacle Detection

Modern PSD systems incorporate obstacle detection sensors — typically infrared light curtains or pressure-sensitive door edges — that detect if a person or object is caught in the closing door gap. If an obstacle is detected, the PSD reopens, the train is held, and the obstacle must be cleared before the sequence can restart. On automated lines, an alarm is sent to the control centre and, on GoA4 systems, a CCTV check may be automatically triggered.

Platform Screen Doors and Automation: The GoA Relationship

PSDs are directly linked to the Grade of Automation (GoA) of a metro system. The four GoA levels, defined by IEC 62290 and widely referenced in metro procurement, are:

GoA Level	Operation	PSDs Required?	Examples
GoA 1	Manual — driver controls traction, braking and doors	Optional	Most conventional metros
GoA 2	Semi-automatic — ATO drives the train, driver supervises and manages doors and emergencies	Recommended	London Jubilee Line, Paris Metro Line 1
GoA 3	Driverless operation — no driver, but staff on train for passenger management and emergencies	Required	Copenhagen Metro, Dubai Metro
GoA 4	Fully unattended — no staff on train; remotely monitored from control centre	Required	Singapore MRT (driverless lines), Paris Line 14

At GoA 3 and GoA 4, PSDs are essential because there is no driver at the front of the train to observe the platform before departure. The PSD interlock system replaces the driver’s visual check — the train receives its departure clearance only when the PSDs confirm all doors are closed and the platform is clear.

Benefits of Platform Screen Doors

Benefit	Detail
Passenger safety	Eliminates falls onto the track — accidental, intentional, or caused by crowding. Singapore MRT reported a significant reduction in track intrusion incidents after PSD installation. Networks with PSDs report near-zero platform-edge fatalities.
Energy efficiency (full-height)	In underground stations, full-height PSDs seal the platform from the tunnel, allowing the platform HVAC system to operate without conditioning the entire tunnel volume. Studies on the Hong Kong MTR found energy savings of 15–25% on platform cooling costs after PSD installation.
Service reliability	Objects dropped or thrown onto the track — a frequent cause of delays — are blocked by full-height PSDs. Luggage, rubbish, and personal items cannot reach the live rail area. This reduces track intrusion incidents and the delays caused by stopping trains and clearing the track.
Noise reduction	Full-height PSDs significantly reduce train noise on the platform — particularly relevant for stations with high ambient noise or where platform noise has been identified as a passenger comfort issue. The sealed environment absorbs traction, wheel-rail, and braking noise from the tunnel.
Enables automation	As described above, PSDs are a prerequisite for GoA 3/4 operations. They eliminate the need for a driver’s visual platform check before departure, enabling fully driverless operations that can increase service frequency while reducing operating costs.
Piston effect management	Trains in tunnels create a piston effect — pushing a column of air ahead and creating uncomfortable pressure changes and draughts on platforms. Full-height PSDs buffer this effect, significantly improving platform comfort particularly at busy interchange stations.

Major Metro Systems with Platform Screen Doors

System	City	PSD Type	Coverage	Note
Singapore MRT	Singapore	Full-height	All stations	World pioneer — PSDs from 1987 opening
Hong Kong MTR	Hong Kong	Full-height	All underground stations	Major energy saving benchmark for tunnel stations
Paris Metro Line 14	Paris	Full-height	All stations (GoA4)	First fully automated line in a major European metro (1998)
Dubai Metro	Dubai	Full-height	All stations (GoA4)	Critical for platform cooling in desert climate
Copenhagen Metro	Copenhagen	Full-height	All stations (GoA3/4)	Driverless from opening in 2002
London Underground	London	Full-height (Jubilee, some DLR)	Jubilee line, Elizabeth line extensions, DLR	Retrofit ongoing — Piccadilly line upgrade includes PSDs
Tokyo Metro	Tokyo	Full-height & half-height	Majority of stations — ongoing retrofit	Largest retrofit programme in the world by station count
Incheon Airport Maglev	Seoul	Full-height	All stations	Integrated with maglev driverless operation

Challenges of Retrofitting PSDs on Existing Lines

While PSDs are standard on new metro systems, retrofitting them onto older lines presents significant engineering and operational challenges:

Mixed rolling stock: Older lines often operate multiple train generations with different door configurations — different numbers of doors, different door widths, and different door positions. A PSD panel designed for one train may not align with another. Solutions include movable platform barriers (MPBs) or — more commonly — standardising the fleet before installing PSDs.
Platform structural capacity: Full-height PSD frames are substantial steel structures. Many older station platforms were not designed to carry this additional load and may require structural reinforcement before PSDs can be installed.
Stopping accuracy: Legacy signalling systems often cannot guarantee the precise stopping accuracy required for PSD alignment. Upgrading to CBTC (typically done simultaneously with PSD installation) adds significant cost and complexity.
Operational continuity: Retrofitting PSDs on a busy operational metro requires working overnight in narrow maintenance windows. Tokyo’s retrofit programme, the world’s largest, has been running for over a decade and works through stations progressively during nightly closures.
Emergency evacuation: PSDs must have provisions for emergency passenger evacuation from the train onto the track — typically via dedicated emergency access gates or breakaway panels in each PSD bay. Emergency procedures must be updated for every station where PSDs are installed.

Frequently Asked Questions (FAQ)

1. Why don’t all metro systems have platform screen doors?

The primary barriers are cost and complexity. For a new metro system, PSDs add approximately 5–10% to the total project cost but are widely considered worthwhile given the safety and operational benefits. For existing systems, retrofit costs are much higher — requiring structural work, signalling upgrades, fleet standardisation, and years of night-time installation work. Some older systems, like parts of the New York Subway and London Underground, have platform configurations that make PSD installation particularly challenging due to curved platforms, variable train stopping positions, and ageing infrastructure. Many systems are progressively installing PSDs as part of broader upgrade programmes — but this takes decades at scale.

2. What happens if a platform screen door fails to open?

PSD systems are designed so that a single panel failure does not block the entire platform. If one PSD panel fails to open, the corresponding train door is also held closed, and passengers use the remaining open doors in adjacent bays. This approach — known as “graceful degradation” — allows the service to continue while the failed panel is logged for maintenance. A failed panel that cannot close is more serious: the train interlock will prevent departure until the fault is cleared or the panel is manually locked out and secured. The relevant train door is also locked closed, and the service continues with reduced door capacity on that bay.

3. Do platform screen doors work on curved platforms?

Yes, but with engineering challenges. On curved platforms, the gap between the train body and the platform edge varies along the train’s length — a problem known as “kinematic envelope intrusion” or the “gap and step” issue. On tight curves, this variable gap can make it difficult to design a standard PSD panel that accommodates the full range of train positions without creating safety gaps. Solutions include curved PSD designs, wider door openings to accommodate variable train positions, and filling the residual gap with flexible rubber edge seals. Some very tight-radius curves — such as certain London Underground stations — are considered too difficult to retrofit with standard PSDs without major platform reconstruction.

4. Are platform screen doors required for driverless trains?

For GoA 4 (fully unattended) operation, PSDs are effectively mandatory — they replace the driver’s visual platform check before departure. Without PSDs, a departing driverless train has no reliable means of confirming that no passenger is in the gap between the train and the platform edge. For GoA 3 (on-train staff but no driver), PSDs are also the norm, as the on-board attendant cannot simultaneously observe the entire train length and manage passenger boarding. GoA 2 (ATO with a supervising driver) does not strictly require PSDs, but many GoA 2 systems install them to improve safety and enable a future upgrade to GoA 3/4.

5. What is the difference between a platform screen door and a platform edge door?

The terms are often used interchangeably, but they technically describe different configurations. A Platform Screen Door (PSD) specifically refers to a full-height floor-to-ceiling system that completely separates the platform environment from the tunnel. A Platform Edge Door (PED) refers to a half-height system — typically 1.2–1.5 m tall — that creates a physical barrier at the platform edge without sealing the environment. In practice, “PSD” is widely used as a generic term for both full-height and half-height systems. The distinction matters primarily in specifications, where the full-height PSD offers energy efficiency and noise benefits that the half-height PED does not.

📖 Related Reading — Metro Systems & Automation
🔌	What is CBTC? Communications-Based Train Control Explained How CBTC enables precise stopping, moving block operations and the automation levels that make platform screen doors possible.	Read More →
🚆	Singapore Cross Island Line: Stations, Map & Phases Singapore’s next major metro expansion — built to the same full-height PSD standard as the original 1987 MRT network.	Read More →

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