Lots of single mode fiber in redundant loops. In longer tunnels you'd have several technical rooms along that hold the loop ends into L3 switches. Within the lo

Lots of single mode fiber in redundant loops. In longer tunnels you'd have several technical rooms along that hold the loop ends into L3 switches. Within the loop you have emergency cabinets spaced 125m. Apart from fire extinguishers and emergency phones (often also our products!) they contain some PLCs and L2 switches that distribute signal and PoE to end point devices such as Evacsound or traffic cameras.