It was a proud day in Medellín, Colombia. Mayor Alonso Salazar smiled as he announced the city's latest accomplishment, and a sycophantic press reported his happy message verbatim. The 12,000 desperately poor people of Medellín's Comuna 13, a shantytown set high atop a steep hill, had spent generations climbing up and down some 530 mountainside stairs to reach their homes. It has always been a 35-minute walk, each way, a challenge for even the healthiest among them. But it is no longer. What did the city do to alleviate the pain of its most destitute? The city of Medellín, long...