The past three days have been spent flying from Japan to the U.S. and back. When we left Japan a couple of days ago, we were assigned Track one for the initial part of the flight, then transitioned to a random route. Air traffic control establishes specific tracks or routes across the oceans to make the flow of aircraft operate in a more orderly fashion. The tracks vary in location from day to day and sometimes throughout the day, and track messages are published giving the latest routes and their effective times. The tracks are usually roughly aligned with the most favorable winds.