Complex sentence: The "third crowd" filled one railroad passenger car and left Staunton, Virginia on May 12 and traveled via Chicago and Marshalltown, Iowa, arriving in Muchakinock on May 15.
Split into simpler sentences: The "third crowd" filled one railroad passenger car. It left Staunton, Virginia on May 12 and traveled via Chicago and Marshalltown, Iowa, arriving in Muchakinock on May 15.