I know is has somthing to do with trains, but a google search didn’t turn up anything defining the term.
From this site.
A line serving one or more stations beyond the point of junction with the main or another branch line
I am not connected with any railroad. However, consider this example. The Illinois Central Railroad was built for the purpose of typing North and South together in the increasingly tense times before the Civil War. One reason was to increase North-South commerce to get hoped-for increased amity.
Anyway, the ICRR main line ran from Chicago to New Orleans. But a branch line was added from Chicago to Omaha, NE. Another branch ran over into Indiana. The Chicago-Omaha branch divides at my old home town and a sub-branch (a twig?) goes up to Sioux Falls, SD, about 80 miles away.
The dividing point between “main line” and “branch line” is, like so many things, pretty arbitrary.
The main East Coast line in Britain goes from the terminal in London (Kings Cross) to Edinburgh (and then beyond).
The 4th stop out from Kings Cross is Alexandra Palace.
At this point there’s a bridge over the main line. The trains that go over it are heading up a separate track.
All the stations on this line serve populations that are tiny compared to the two cities above.
So it’s called a branch line (to distinguish it from the main line).
About 20 years ago they extended the branch line so it rejoins the main line further up. I suppose that could make it a ‘loop’.
To continue the tree theme, the main line is sometimes also called the “trunk line”.
Just think of it like a tree, the main trunk running from important place A to important place B, with smaller lines branching off to more minor locations.
Take a look at the New Haven Line (the red one) of Metro-North Railroad.
The main line goes from Grand Central Terminal up the coast to New Haven, CT. There are three branch lines which connect to the main line. The first goes from Stamford to New Canaan, the second goes from South Norwalk to Danbury, and the third goes from Stratford to Waterbury.
Just wait till you get to siding, spur, and industrial track…