Line
One cycle of a declared service.
The third way a tour comes into existence, and the one that makes a route system a configuration rather than a special case: where a transport task becomes two stops and an errand becomes one, this becomes the line's own list of them. Nothing is suspended on it -- the riders are suspended at their stops, not on the vehicle's work -- so it behaves like an errand in every respect that matters to the protocol: it may be cancelled, and it never passes through IN_PROGRESS.
A vehicle is assigned a line each cycle rather than owning one. That is what keeps a fleet's allocation across its services visible as a dispatching decision, and what puts a decision point at the end of every cycle instead of committing a vehicle to a service for the whole run.
Properties
Where the vehicle must end up.
Where the vehicle must go first. The destination itself for a task with nothing to collect, which is what makes a service task a one-stop tour.
The entity suspended on this task, if any. Null for a task a vehicle raised for itself.