The Tunnel Effect: EN 14067-3 Aerodynamics

Introduction to EN 14067-3

When a high-speed train enters a tunnel, it acts like a piston in a syringe. It compresses the air in front of it, creating a pressure wave that travels down the tunnel at the speed of sound. This wave hits the end of the tunnel, reflects back, and hits the train again. For passengers, this manifests as the uncomfortable sensation of ears “popping.”

EN 14067-3, titled “Railway applications – Aerodynamics – Part 3: Aerodynamics in tunnels,” is the physics manual for managing these invisible forces. It provides the equations to calculate air resistance (drag) in confined spaces and sets the strict medical and comfort limits for pressure variations to ensure that a trip through the Alps doesn’t result in ruptured eardrums.

Snippet Definition: What is EN 14067-3?

EN 14067-3 is a European standard describing the physical phenomena and calculation methods for railway aerodynamics in tunnels. It defines the limits for pressure variations relative to time (to protect passenger health and aural comfort), calculates the aerodynamic drag factor for tunnel energy consumption, and addresses the mitigation of Micro-Pressure Waves (the “sonic boom” effect) at tunnel exits.

The Pressure Wave Phenomenon

EN 14067-3 breaks down the chaotic airflow into predictable wave patterns:

• Entry Wave (Compression): As the nose enters, pressure rises instantly.
• Friction Effect: As the long body of the train moves through, friction with the tunnel walls drags air along, changing the pressure gradient.
• Exit Wave (Expansion): As the tail leaves, pressure drops suddenly.

These waves bounce back and forth (reflect) until they dissipate. If two trains pass each other in a tunnel, their pressure waves superimpose (add up), potentially doubling the load on the windows and the passengers’ ears.

Safety and Comfort Limits

The standard sets two distinct types of limits:

1. Health Limits (Safety)

This is the absolute maximum pressure change the human body can withstand without damage (e.g., lung or ear injury).

• The Limit: The maximum peak-to-peak pressure change anywhere in the train must not exceed 10 kPa over the duration of the tunnel passage.
• This ensures that even if the air conditioning sealing fails, passengers are safe.

2. Aural Comfort Limits (Quality)

This is about keeping passengers happy. It limits how fast the pressure changes.

• Criterion: Defined pressure changes (e.g., < 800 Pa within 4 seconds) to ensure that most passengers do not feel the need to swallow to clear their ears.
• Solution: Modern high-speed trains (like the ICE or TGV) use “pressure-sealed” carriages where the HVAC system closes valves to isolate the interior from the sudden external spikes.

The “Sonic Boom” (Micro-Pressure Wave)

EN 14067-3 addresses a specific environmental pollution issue for high-speed lines.

When the compression wave created by the train entry reaches the other end of the tunnel, it radiates out into the valley as a loud bang or low-frequency boom. This can shake windows in houses kilometers away.

• Mitigation: The standard discusses the use of Tunnel Hoods (flared entrances) with slits. These structures allow the air pressure to release gradually rather than instantaneously, killing the boom.

Comparison: Tunnel (Part 3) vs. Open Air (Part 4)

The physics change completely when walls are added.

Operational Relevance

Speed Limits: Tunnel aerodynamics are the primary reason why trains often slow down in tunnels. The aerodynamic drag (energy cost) increases with the square of the speed, but the pressure discomfort increases even faster. EN 14067-3 calculations are used to determine the Maximum Tunnel Speed to stay within the 10 kPa safety limit.