main line

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

• noun A principal section of a railroad line.
• noun Slang A principal and easily accessible vein, usually in the arm or leg, into which a drug can be injected.

from the GNU version of the Collaborative International Dictionary of English.

• noun Ahe principal route of a transportation system, such as a railroad or bus line.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The principal route or line of a railway.
• noun fishing In longline fishing the central line to which the branch lines with baits are attached.
• noun slang A principal vein into which a drug can be injected.

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

• noun the principal route of a transportation system