Circular Rail Lines

https://en.wikipedia.org/wiki/Circle_route

https://www.webuildvalue.com/en/infrastructure-news/metro-urban-mobility.html

Some examples...

https://en.wikipedia.org/wiki/Berlin_Ringbahn

https://en.wikipedia.org/wiki/Chicago_%22L%22#Circle_Line

https://en.wikipedia.org/wiki/M3_(Copenhagen_Metro)

 https://en.wikipedia.org/wiki/Circle_line_(London_Underground)

https://en.wikipedia.org/wiki/U3_(Hamburg_U-Bahn)

https://en.wikipedia.org/wiki/Moscow_Central_Circle

https://en.wikipedia.org/wiki/Ring_Line_(Oslo)

https://en.wikipedia.org/wiki/Paris_M%C3%A9tro_Line_15 , https://upload.wikimedia.org/wikipedia/commons/e/ea/Plan_Lin%C3%A9aire_M%C3%A9tro_Paris_Ligne_15_%28final%29.svg

https://en.wikipedia.org/wiki/City_Circle, Sydney.