Doing a Quick Drive I noticed a lack of other trains on the route. I reported this to DTG Support - and got a reply "Thank you for bringing this issue/s to our attention. Under testing this issue was reproducible. We value your feedback, and please accept our apologies for any inconvenience caused. The issue/s below have been documented and flagged for further investigation by the development team. Issues will be prioritised upon receipt based on how severely they affect gameplay. NEC: New York-New Haven Route Add-On - When playing a quick drive scenario there are no AI driven trains on the route. The best way to keep up to date with this issue and more is to monitor our official community channels for updates."
It's been like that since it was released. I don't understand the reason that no AI was placed. It's pretty annoying and makes an otherwise enjoyable route to run on, boringly dead.