Manning the rail is a method of saluting

Salute

A salute is a gesture or other action used to display respect. Salutes are primarily associated with armed forces, but other organizations and civil people also use salutes.-Military salutes:...

 or rendering honors used by naval vessels. The custom evolved from that of manning the yards, which dates from the days of sail. On sailing ships, men stood evenly spaced on all the yards (the spars holding the sails) and gave three cheers to honor distinguished persons. Today the crew are stationed along the rails and superstructure

Superstructure

A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships...

 of a ship when honors are rendered.

The United States Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 prescribes manning the rail as a possible honor to render to the President of the United States

President of the United States

The President of the United States of America is the head of state and head of government of the United States. The president leads the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces....

 and for the heads of state of foreign nations. A similar but less formal ceremony is to have the crew "at quarters" when the ship is entering or leaving port.

Manning the rail is also the traditional way to honor the USS Arizona Memorial

USS Arizona Memorial

The USS Arizona Memorial, located at Pearl Harbor in Honolulu, Hawaii, marks the resting place of 1,102 of the 1,177 sailors killed on the USS Arizona during the Attack on Pearl Harbor on December 7, 1941 by Japanese imperial forces and commemorates the events of that day...

 when it is passed by all U.S. Navy, U.S. Coast Guard, and U.S. Merchant Marine vessels. More recently, as foreign military vessels are entering Pearl Harbor for joint military exercises, foreign troops have participated in the traditional manning the rails. Other notable instanced occurred on July 24th 1997 when the guided missile destroyer and frigate rendered honors to the Constitution

USS Constitution

USS Constitution is a wooden-hulled, three-masted heavy frigate of the United States Navy. Named by President George Washington after the Constitution of the United States of America, she is the world's oldest floating commissioned naval vessel...

during her 200th Birthday celebration, and on September 14, 2001, when the crew of the destroyer FGS Lütjens manned the rails as they approached the destroyer USS Winston Churchill and displayed an American flag and a banner reading "We Stand By You".