Complex sentence: In the late 1880s, Ingalls and the Vanderbilts gathered a group of railroads between Cairo and Danville, Illinois; added to them the St. Louis, Alton & Terre Haute; and then added them all to the Big Four.
Split into simpler sentences: In the late 1880s, Ingalls and the Vanderbilts gathered a group of railroads between Cairo and Danville, Illinois. After adding the St. Louis, Alton & Terre Haute to that group, they added the group to the Big Four.