What causes cable cars, tunnels, and other line-shaped objects to appear?

The system keeps track of tasks that are marked "in progress" as you continue from day to day. When you mark a task as "in progress" and do not mark the task as "complete" on the day in which it is due, the task will be automatically marked as "incomplete". You will then see a cable car, tunnel, or some other line-shaped object appear on your land.