trunk line

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A direct line between two telephone switchboards.
• n. The main line of a communications or transportation system.

from the GNU version of the Collaborative International Dictionary of English

• n. the main line of a railway, canal, or route of conveyance.

from The Century Dictionary and Cyclopedia

• n. In electricity, lines or wires which connect two telephone-exchanges, as distinguished from the wires which connect an exchange with its subscribers.
• n. The main line, as of a railway or canal, from which branch-lines diverge.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. a telephone line connecting two exchanges directly
• n. line that is the main route on a railway