The GN built a bridge across the Columbia River, near the town of Wenatchee, WA in 1892. It was designed to carry two 122 ton locomotives pulling a uniform train of 4000 lbs per lineal foot.
In 1926 GN went about bringing the bridge up to the "highest standards" and doubling the loading of the original design.
The method choose, in simplest terms, was to build a bridge around the original bridge. If you can find a copy, Volume 78 (May 1 1926) of Railway Review has a very through article on the upgrade construction.
The bridge is still in service for the BNSF, more than a hundred years from it's original construction.
Here then is an aerial video of the Rock Island Bridge.
BNSF Stack Train crossing the Rock Island Bridge