Gantry: "a platform made to carry a traveling crane, supported by towers or side frames
running on parallel tracks." This yellow gantry sits atop piers (columns) that support
the elevated platform for a light rail line. The rail line will connect downtown
Seattle with Seattle-Tacoma International Airport.

A crane on the gantry is used to lift pieces of the platform into place so they can be
connected to existing pieces. When the section between two piers is completed, the gantry
shifts ahead until it makes contact with rollers on the next pier, forming a bridge. Thus
the whole structure slowly moves forward from one pair of piers to the next. It is long
enough to span the distance inside three piers. The thing is magnificent.

These pictures were taken in Tukwila, a city that abuts the south edge of Seattle.
The gantry is on the west side of Interstate 5 and the north side of state highway
518 (the westward extension of Interstate 405). The gantry is moving northward from
the airport toward Seattle. Next year it should reach the scene pictured here:
http://www.pbase.com/listorama/image/61512842 , moving from right to left across the tracks.
.