<human>:  Why is the London Underground, or the Tube as it is known, called the Tube?
<bot>: The Tube is called the Tube because it is a tube-shaped railway system. It is also known as the London Underground. The London Underground is the oldest underground railway in the world. It was built in 1863, and is still in use today. It is also one of the busiest underground railways in the world, carrying over 2 billion passengers every year.