noun
four-track rail line; quadruple track
Railway term for a line with four tracks, typically two in each direction, allowing express and local services to run separately.
この区間は複々線化されて、快速と各駅停車が別の線路を走る。
This section has been quadruple-tracked, so express and local trains run on separate tracks.
Standard kanji spelling using the iteration mark 々.
Alternative spelling without the iteration mark; less common but still used.
複線 means a double-track line (one track in each direction), while 複々線 refers to a quadruple-track line.
Compound of 複 (ふく, 'double') repeated with the iteration mark 々, plus 線 (せん, 'line'), literally 'double-double line'.