trunk line

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun In electricity, lines or wires which connect two telephone-exchanges, as distinguished from the wires which connect an exchange with its subscribers.
• noun 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.

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