Thousands of migrants riding atop railway cars in Mexico this week or waiting in mile-long lines by the tracks to hitch a ride to the U.S. have triggered the closure of one U.S. border crossing and forced Mexico’s largest railroad to suspend dozens of freight trains. The dangerous jungle crossing has been reduced from over a week to as little as two days for the strongest walkers, by a network of guides, established camps and rudimentary trail markers that start in the Necocli, on Colombia’s side of the gap.