Train lines are not useful until suddenly they become extremely powerful. It's really just a matter of base size.Yes, trainlines in 0.13 might not be as usefull for smaller/vanilla ressource maps. Back in 0.12 trainlines offered the conditions that are now vanilla.
As soon as you have several trains taking the same route, it's already nice to have the grouping visible in the UI. It shows you the number of trains, and makes it easier to add a new train to an existing line, without having to copy-paste the settings from another train.
The real power comes when you use the signal to go to a station directly. That's the killer feature of Smart Trains that Vanilla Factorio does not provide.
You can also read cargo of a train, another feature that is still missing in Vanilla and is requested ALL THE TIME by players.Lol: Everyone is doing all these fancy things, and here I sit, just using it to make sure my trains have enough fuel