A railway strike would have spelled trouble for importers bringing goods through U.S. ports.
Maersk said its chairman, Michael Pram Rasmussen, would step down effective March 28.
Successful passage of first ever 10,500-plus TEU container ship signals a new era of mega-ship transits.
Performing the 500th transit was the container vessel YM Unity.
The scope of the agreement applies only to trade lanes between the U.S. and other nations.