noun
uncompleted railway line
Refers to a railway line that was planned or partially constructed but never finished. Used in historical or planning contexts.
この未成線は、戦前に計画されたが実現しなかった。
This uncompleted railway line was planned before the war but never realized.
廃線 refers to a railway line that was once operational but has been abandoned or closed, whereas 未成線 was never completed or opened.
Compound of 未成 (みせい, 'uncompleted') and 線 (せん, 'line'). The exact historical derivation is uncertain, but the term is conventionally associated with railway planning and history.