main line; trunk line
Refers to the primary route in a railway, road, or utility network, as opposed to branch lines or local lines.
この電車は本線を走っています。
This train is running on the main line.
水道の本線が工事中です。
The water main is under construction.
I merged onto the main line from the highway ramp.
This train runs on the main line.
This branch line splits off from the main line.