While I agree that 60 trains a day seems a bit much for single track w/o sidings, note that it works out to a train every 30 minutes each way (4/hour) * 15 hour operating day. So, say, 0600-2100. That's not exactly NY subway service, and matches or improves on the headways of SC's main line buses. Certainly would not require double-tracking the line; perhaps adding a few strategically placed sidings. And of course signals - which could end up (with PTC) the most expensive part of the project.