JR Yamanote line is a circular or loop line in the middle of Tokyo not unlike Circle line for London Tube.

Naturally circular line will be the most popular since other lines are moving radially from city center thus travelling between two nearby places can take longer by changing trains if they're not on the same the line.

Case in point, Bordeaux city in southwest of France currently has no circular tram line (yet) hence most of the public transport users are relying on the buses to go to the nearby places that are close but on different tram lines.