begin
Sends the vehicle to to and hands back the queue to wait in.
A second call while a journey is under way is a redirection, not an error: the target changes and the running step chain re-plans from wherever the vehicle is when its current step completes. The odometer keeps growing across it, because a vehicle that turns round has still covered the ground it covered.
Return
the queue to suspend waitingFor in, or null when the vehicle was already there