noun, noun which may take the genitive case particle 'no'
on a line; on the line
Literal spatial meaning: located on a line, such as a drawn line, a route, or a boundary line.
点が線上に並んでいる。
The dots are lined up on the line.
この線上をまっすぐ歩いてください。
Please walk straight along this line.
noun
Figurative use: at a critical dividing point between two states, often with 〜の線上 or 〜線上にある.
彼の提案は実現可能性の線上にある。
His proposal is on the borderline of feasibility.
交渉は決裂の線上にある。
The negotiations are on the verge of breaking down.