i checked the link first, and i am glad i did--i was going to say the only reference i could think of was a rail/subway line.

sometimes called the main line in Northeast (maybe even in all of US) --most notably in Pennsy, where the main line helps define not just the railway, but the upper crust of society!

LIRR has several through lines, the original line (port washington line) and the Jamaica line -(to babylon) and many branches. some lines terminate in jamaica, some in hunters point, some at atlantic avenue, but the through line continues to Penn station. (which is also the terminal for the LIRR.

my other obsession