noun
1. : a system (as for transportation) handling long-distance through traffic : main line
like any highway system, the federal airways have their main trunk lines as well as feeder routes — Monsanto Magazine
specifically : one of the railway systems whose main lines run from Chicago to New York
2.
a. : a main supply channel (as a natural-gas main)
b. : a direct link (as a telephone circuit between two switchboards)