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.
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.
This road continues along the coastline.
This train runs along the coastal railway line.
The beautiful coastline stretches on for kilometers.