Railroad tunnels are underground passages designed for trains to travel through mountains, hills, or urban areas. They help reduce travel time and avoid obstacles, making train routes more efficient. These tunnels are engineered to support heavy loads and ensure safety for both passengers and cargo. Constructed using various methods, such as cut-and-cover or bore tunneling, railroad tunnels can vary in length and diameter. Some famous examples include the Gotthard Base Tunnel in Switzerland, which is one of the longest in the world, and the Channel Tunnel connecting England and France.