<p>The urban sites of sport mega-events, such as the World Cup or the Olympic and Paralympic Games, have become stages for experimenting with extraordinary security and safety procedures. These procedures are designed to demonstrate the host country’s capacity to mobilize in the face of an extreme level of terrorist threat while also providing a safe and welcoming environment for attendees under the Olympic principles of peace and equality. The exceptional nature of the threat associated with such events means that these experiments not only amplify existing systems and procedures but also challenge their structural standards, strategies, and limits, allowing for significant changes to be considered post-event, based on the outcomes of these experiments. In this article, we argue that the ability of such experimentation to serve as leverage for urban transformation makes it part of the legacy of mega-events. To illustrate this, we analyze the security mobilization in the railway transport spaces of the Parisian region during the 2024 Summer Olympic and Paralympic Games. We examine how experimental practices, developed as part of an extraordinary counterterrorism response, have challenged the spatial coordination of security and responsibility in multimodal stations, interinstitutional cooperation, the role of passengers in counterterrorism efforts, and the ethical boundaries of associated digital practices. We also explore how framing the response’s elements as experiments can help optimize the policy learning process and optimize the long-term implementation of the lessons learned during it.</p>