noun
parallel railway lines
Refers specifically to two or more railway tracks running side by side without intersecting.
この区間は並行線になっていて、上下線が別々の線路を走る。
This section has parallel tracks, with inbound and outbound trains running on separate lines.
平行線 is the general term for parallel lines in geometry or metaphorically for parallel paths that never meet. 並行線 is a specialized railway term for parallel tracks.
Compound of 並行 (parallel, side by side) + 線 (line). The kanji 並 means 'line up' or 'parallel', while the variant 併 can mean 'join' or 'alongside'.