noun
coastline; shoreline
Refers to the line where land meets sea; used in geography, maps, and general description.
日本の海岸線はとても長い。
Japan's coastline is very long.
地図で海岸線をなぞってみた。
I traced the coastline on the map.
noun
coastal railway line
Specifically a railway line that runs along the coast; often used in station names or route descriptions.
海岸線を走る電車から海が見える。
You can see the ocean from the train running on the coastal line.