This is the 50' Thru Girder Bridge Single Track Model Train Bridge in HO Scale by Micro Engineering.
These kits contain all the details and interior structure found on prototype bridges, such as lateral bracing, crossbeams with knee braces, stringers, rivet plates, bridge shoes and extensive molded-in detail including rivet heads. HO Code 83 Bridge Flex-Track is included with each kit. The bridges can be modeled with square or rounded ends on the girders -- both types are found on the prototype. They can also be modeled as open-deck bridges using the "concrete" deck included in each kit. Additional Through-Girder Bridges and/or Deck Girder Bridges can be combined, along with Bridge Supports #255-80175 (all sold separately) to form longer custom bridges.