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Why does the Channel Tunnel include a central service tunnel?

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English Channel The Baltic and North seas and the English Channel.

The Channel Tunnel includes a central service tunnel primarily to facilitate essential operational and safety functions. Specifically, this third tunnel is utilized for:

Ventilation: Ensuring proper airflow throughout the rail system.1
Maintenance: Providing access for crews to perform necessary upkeep on the infrastructure.1
Emergency Evacuation: Serving as a secure route for safety and security purposes in the event of an incident.21

By separating these functions from the two main rail tunnels, the design enhances the overall safety and efficiency of the 50-km (31-mile) link between England and France.21