In my dreams it would be a priority based system, but in reality it’s much more basic.
2 tracks become backed up later in game, I think last game had something like 60ish 10car trains moving around the map. I double up the lines manually. Farther lines use the inside tracks (express/fast lines) and local ones or things that split off use the outer (local/slow) tracks. This keeps the longer distance trains from needing to stop as frequently waiting for locals to get clearance.