1. 支线：干线 trunk line; main line | 支线 branch line | 铁路专用线 railway special line

2. 岔线：section line 段管线 | branch line 岔线 | special line 特别用途线

3. 支管：branch line tester 支管测试器 | branch line 支管 | branch loop 分路回路的一个分支

4. 分枝线：branch instruction 分枝指令 | branch line 分枝线 | branch of a curve 曲线的分枝

1. （铁路）支线
    A branch line is a railway line that goes to small towns rather than one that goes between large cities.

1. a railway line connected to a trunk line

    Synonym: spur trackspur