trunk line

plural trunk lines

1. rail transportA main railway line which usually forms the backbone of a rail network.

   But the states were generally concerned with the location of trunk lines and left it to cities and counties to worry about branch lines.

   1987, William H. Rehnquist, The Supreme Court

2. The main line of a canal, from which other sections diverge.
3. A main telephone line.

   Then he put a trunk line phone call through to the nearest township and asked the Police to send someone out as soon as possible.

   1943, H. Lorna Bingham, The Lost Tribe, Sydney: Winn and Co., page 13, column 2