noun
existing line; line in operation
Technical term used in railway, power, or telecommunications contexts to refer to a line that is already built and in service, as opposed to a planned or under-construction line.
新線は既設線に接続する予定です。
The new line is scheduled to connect to the existing line.
既存 is a more general adjective meaning 'existing', used for various facilities or systems, whereas 既設線 specifically means an existing line (railway, power, etc.).
Compound of 既設 (kisetsu, 'already established') + 線 (sen, 'line'). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.