Modern railway systems are designed to avoid a train from being stopped in a tunnel by an onboard fire. In the unlikely event that firefighting intervention is necessary in a tunnel, firefighters can access the tunnel via intervention points, which include stations, intermediate shafts and portals. Establishment of the maximum distances between intervention points has a significant impact on project feasibility, cost and planning due to the constraints of constructing frequent intervention points in a city environment. By mapping out the stakeholders, their actions and requirements, an approach to tunnel design has been developed to mitigate impacts of longer intervention distances on firefighter safety. The fire engineering design team engaged with the fire brigade and rail operator in a series of interactive workshops. During these workshops it became clear that the physiological impact on firefighters during intervention may be a limiting factor in determining the maximum distance between intervention points. A literature review was carried out on research relating to the physiological impacts on firefighters undertaking strenuous activities in a tunnel environment, including consideration of the impact of elevated temperature, gradients, time and distance. This research validated that activities undertaken by firefighters in response to a fire incident can put them at risk of heat stress and fatigue.