The 606

This guide to The 606 in Chicago starts with a single plant along the trail's edges: milkweed, the only thing monarch caterpillars will eat. It traces why the butterflies' migration toward Mexico depends on finding it here, and follows the elevated path's earlier life as a freight railway, built after a great fire to lift trains above street level. For more than a decade the line sat quiet and rusting before anyone touched it. Look along the edges as you walk for the orange clusters of butterfly weed.