Railway undertakings and infrastructure managers are responsible to continuously control the risks associated to their own activities under all conditions. Of course, it applies to clearly identified risks railway undertakings and infrastructure managers are responsible for (including risks where they have shared interfaces with other entities). Indeed, railway undertakings and infrastructure managers cannot be reasonably asked to address risks that cannot be known or even that can be known but are outside their sphere of control (e.g. air plane crashing on the track).