I have a long distance raw mats line and the mine kept dropping the line connection. I think it has something to do with the fact that it was being visited too rarely by a train? Not entirely sure. I now have 4 trains running and it didn’t do it since years.
The best thing you can do is to add more vehicles to ensure frequency, and create demand at the factory (100+) by supplying nearby cities. It’s quite difficult with slow vehicles but that’s how I am trying to get it running. I don’t have a lot of success either, had to send every train to depot to electrify the line and now about 5 years passed and the industries still don’t produce enough for me to go into + money… 🙂