This is the Tall Steel Viaduct Kit 320' Long Model Train Bridge in N Scale by Micro Engineering.
Tall steel viaducts are some of the most spectacular and interesting types of bridges. They are used by railroads to span wide and/or steep valleys, often replacing wooden trestles. These HO and HOn3 Tall Steel Viaducts are available in three base kits, three length extension kits, a height extension kit which adds a 4th and 5th story, and a tower kit. The various kits allow you to design and build your own custom bridge of any length from one to five stories high, with full towers or bents, and straight or curved. A 10-page instruction sheet with over 35 photos and illustrations is included. These bridges are beautifully detailed and are often the centerpiece structure on modeler's layouts.