From Longman Dictionary of Contemporary Englishmain lineˌmain ˈline noun [countable] TTTan important railway that connects two cities the main line to Moscow —main-line adjective [only before noun] a main-line station
Examples from the Corpus
main line• Electrification continued apace during the decade with the major investment on the East Coast main line and in East Anglia.• It is difficult to imagine that this was once Britains first electric main line.• Also, standards of service on many main line routes were improved considerably, particularly the inter-urban routes.• Don Robey built an empire worth millions in a city far removed from the main line of entertainment.• Now the main line up the Meuse was severed by the enemy astride it at St. Mihiel.• Well I the uh main line dancers whoo. 