Sushi train is called Kaiten-sushi (回転寿司) which means Rotation Sushi, is a very popular type of Sushi restaurants. In the restaurant, instead of waiter/waitress carrying sushi to customers, fresh sushi rotates through the restaurant on a conveyor belt.
The popularity is due to the ease and pricing. It makes it easy for customers to enjoy sushi as all they need to do is to enter the restaurant, take a seat, then choose the sushi to eat. As there are less staff to manage the restaurant, the pricing of each sushi tend to be cheaper than standard sushi restaurants.