The Railway Exchange Building is a 84.4 m (277 ft), 21-story high-rise office building in St. Louis, Missouri, which was connected to the now-defunct One City Center mall (now being redeveloped as the Mercantile Exchange project). The 1914 steel-frame building is in the Chicago school architectural style, and was designed by architect Mauran, Russell & Crowell. The building was the city's tallest when it opened, and remains the second-largest building in downtown St.