At the end of the first decade of the 21st century, downtown Tucson underwent a revitalization effort by city planners and the business community. The primary project was Rio Nuevo, a large retail and community center that has been stalled in planning for more than ten years. Downtown is generally regarded as the area bordered by 17th Street to the south, I-10 to the west, and 6th Street to the north, and Toole Avenue and the Union Pacific (formerly Southern Pacific) railroad tracks, site of the historic train depot and "Locomotive #1673", built in 1900. Downtown is divided into the Presidio District, the Barrio Viejo, and the Congress Street Arts and Entertainment District. Some authorities include the 4th Avenue shopping district, which is set just northeast of the rest of downtown and connected by an underpass beneath the UPRR tracks.
What is the southern edge of Tucson's downtown? (If the question is unanswerable, say "unanswerable")
17th Street