A main line is the principal route between major places. Travel on the main line.
It gets the investment, the fast trains and the electrification, in that order. A main line closure diverts everything onto routes that were never built for it.
1nounmain lineThe principal route between major places.
“The main line carries two hundred trains a day.”
main line is a line.
mainline comes from main + line.
Etymology from Wiktionary, CC BY-SA
Practised in Railways