once you have connected multiple cars together for a consist, you can’t then run around your consist and connect to the other end. Unfortunately, that is a limitation of the game engine. (It is a farming game after all, never meant for trains like this haha) Nothing I can do to change that sadly. So the way that I do it, is once I build a consist. If I’d like to do a runaround, I hold the “control” key and disconnect all of the rail cars so that they are just sitting there disconnected, then runaround, then reconnect them all from the other end.