If everyone had kept quiet, it could have been the most valuable parking spot on earth. Convenient only to the careworn clothing stores clustered in the southern end of downtown Nogales, Ariz., it offered little to shoppers, and mile-long Union Pacific (UNP) trains sometimes cut it off from much of the city for 20 minutes at a time. But the location was perfect: In the middle of the short stretch of East International Street, overshadowed by the blank walls of quiet commercial property, the space was less than 50 feet from the international border with Mexico.