What is Branch Line?

Legal Definition
A branch line is a secondary railway line which branches off a more important through route, usually a main line. A very short branch line may be called a spur line. David Blyth Hanna, the first president of the Canadian National Railway, said that although most branch lines cannot pay for themselves, they are essential to make main lines pay.
-- Wikipedia
Legal Definition
A secondary railroad line branching off from a main line.