Rail tunnels are underground passages designed for trains to travel through mountains, cities, or other obstacles. They help reduce travel time and improve safety by allowing trains to avoid surface-level traffic and natural barriers. Rail tunnels can vary in length and diameter, depending on their purpose and location. Constructing rail tunnels involves advanced engineering techniques to ensure stability and safety. Engineers use methods like tunneling machines and explosives to create these passages. Notable examples of rail tunnels include the Channel Tunnel connecting England and France and the Gotthard Base Tunnel in Switzerland, which is one of the longest rail tunnels in the world.