Final answer:
The longest subway system in the world is the London Underground, also known as the Tube.
Step-by-step explanation:
A subway system is an underground or elevated railway network with trains running on fixed tracks. Found in major cities globally, subways provide efficient mass transit, alleviating traffic congestion. Passengers access stations via entrances, escalators, or elevators. Subway systems contribute to urban mobility, connecting diverse neighborhoods and facilitating daily commuting.
The subway system referred to in the question is the **London Underground**, also known as the **Tube**. It is the longest subway system in the world, with about 244 miles of track. The London Underground was first opened in 1863 as the Metropolitan line, making it the world's first subway system.