It is unknown how Cassatt managed to avoid the Pennsylvania militia draft during the Union mobilization in this period but in 1864, Cassatt was transferred to Renovo, Pennsylvania, as a resident engineer to work on the middle division of the Philadelphia and Erie railroad. In 1866, Cassatt became superintendent of motive power and machinery for the Oil Creek and Allegheny River Railway, recently reorganized in 1864 as the Warren and Franklin Railroad which was growing rapidly due to the discovery of oil in the region and coal mining.