Noun
(rail transport) A main railway line which usually forms the backbone of a rail network.
Coordinate term: mainline
The main line of a canal, from which other sections diverge.
A main telephone line.
Source: en.wiktionary.orgA new train on a new line The new service, named Shinkansen (meaning new trunk line) would provide a new alignment, 25% wider standard gauge, continuously welded rails between Tokyo and Osaka using new rolling stock, designed for convert. Source: Internet
The trunk line consists of many fiber optic cables bundled together to increase the capacity. Source: Internet