Subway tunnels are underground passages designed for trains that form part of a city's public transportation system. These tunnels allow subway trains to travel quickly and efficiently beneath busy streets, reducing traffic congestion above ground. They are typically constructed using heavy machinery and can be lined with concrete to ensure stability. The design of subway tunnels varies depending on the geography and urban layout of a city. Some tunnels are straight, while others may curve to navigate around existing buildings or natural features. The construction of these tunnels often involves careful planning to minimize disruption to the surrounding area and ensure safety for both workers and commuters.