In 1859, Rosengarten witnessed abolitionist John Brown's famous raid on Harpers Ferry, Virginia. Rosengarten had been traveling at the time as a guest with the Directors of the Pennsylvania Railroad, his father being one of the directors. His train stopped short of Harpers Ferry, he walked into town, and witnessed the attack made by Lee's men on the engine house where John Brown had sought refuge. He later saw John Brown lying wounded. Rosengarten wrote a vivid account of what he had witnessed, that was published in the "Atlantic Monthly" in 1865.