To follow a dramatic piece of the Pony Express route in central Wyoming, head west from Casper and Mills, Wyo., on Zero Road, through low hills at Emigrant Gap and past the Poison Spider School. About a mile west of the school, turn left on Natrona County 319, a dirt road that follows the Oregon Trail/Pony Express route closely for about 30 miles. Take this trip in dry, clear weather between about June 15 and October 1. The county road joins State Highway 220 about 15 miles east of Independence Rock. The trail crosses the whole state. Detailed trail guides are available at the National Historic Trails Interpretive Center in Casper, Wyo.