Secondary line of a railroad.

a railway line connected to a trunk line

a line that starts from an existing line and carries its signal to the input port of a block

After the Lancaster and Carlisle line was leased to the LNWR in 1859, the route from Euston to Carlisle became the "main line" and lines off it, however important (e.g. the Manchester and Birmingham line from Crewe to Manchester) were branches. More generally, at any junction the most important through line is the main line and any line off it a branch.

A line serving one or more stations beyond the point of junction with the main or another branch line.

A rail line which serves one or more stations beyond the junction of the main line or another branch line. A feeder line which brings freight to main lines.

A branch line is a relatively minor railway line which branches off a more important through route.