Subway tunnels are underground passages designed for trains that transport passengers in urban areas. These tunnels are typically constructed using heavy machinery and can be made of concrete or steel. They connect various stations, allowing for efficient travel across cities while minimizing surface congestion. The design of subway tunnels varies depending on the geography and infrastructure of the city. Some tunnels are straight, while others may curve or include steep grades. Safety features, such as ventilation systems and emergency exits, are essential to ensure passenger safety during operation. Major cities like New York City and London have extensive subway systems with intricate tunnel networks.