We start in darkness. After fighting our way through the dingy, low-ceilinged, crowded waiting room that serves as New York City’s current Pennsylvania Station, we pull out through a graffitied tunnel that follows one of the oldest roadbeds in America. Freight trains once clattered along open tracks here, spewing smoke within a few dozen yards of the mansions along Riverside Drive and attracting one of the most dangerous hobo encampments in the country, before it was finally all buried beneath a graceful park in the 1930s. Today, we emerge into sunlight for the first time in Harlem, following a…