This is a Code 55 N Scale Double Track Bridge Kit from Atlas.
One Double Track Bridge Kit, Code 55 N scale
The through plate girder bridge is based on the prototype that sported construction of a bridge deck supported between two plate girders. This type of construction gives it the easily identifiable 'U'-shape. The construction depth on this type of bridge was much less than on other bridges, providing a minimal change of height while clearing terrain. It was a common sight on railroads.