The railroad line now known as the First Sub started as the Chicago & Rock Island, chartered by the Illinois legislature to build from Chicago, IL, to the Mississippi River at Rock Island, IL. Started in October of 1952, the line reached Joliet that same month. Nearly 18 months later, the line was complete, with the connection considered complete with trains running the entire length in late February of 1854.

While the line was being built, a connecting road, the Mississippi & Missouri, was incorporated in 1853 to build the next segment - from the Mississippi River at Davenport, IA, westward across Iowa, ideally to the Missouri River on the opposite side of the state. The M&M completed Davenport to Wilton and the branch to Muscatine by 20-Nov-1955. The segment from Wilton Jct. to Iowa City, however, is part of one of the most often retold stories in Iowa railroading. Seeing the Muscatine branch getting greater priority and the M&M getting bogged down in their westward expansion, the citizens of Iowa City posted a $50k bonus for completion of the Wilton-IC segment before midnight of 31-Dec-1855. During that day, rails were still some quarter of a mile short of the goal, and the temperature was reportly around -30 degrees Fahrenheit. History records that even though the rails were completed with a bit of time to spare, the first locomotive suffered some freeze-up in its mechanics and had to be towed into the station using chains, pry-bars, men, and horses. Regardless, the builders made their deadline, and that no doubt fueled the further westward expansion of the line. (Further west of here is IAIS's Third Subdivision - see its page for additional history.)

During the M&M's construction, and just marginally after the C&RI reached its western namesake, construction started on the first railroad bridge over the Mississippi. The bridge was a wood and iron Howe truss design with a swing span in the middle (on the west side of Arsenal Island), set on cut stone piers, and took just over two years to complete. Started on 1-Sep-1954, the bridge carried its first train on 22-Apr-1956. Two weeks after opening, the steamboat Effie Afton struck the bridge in what can only be described as an interesting accident, setting it afire and destroying parts the structure (along with the boat). Throughout the structure's construction, legal questions were raised about the right of a railroad to bridge a major navigable waterway, but the ensuing court cases settled these issues. Eventually taken all the way to the Supreme Court of the United States in 1962, the railroad won the case and the right to keep their bridge in operation... with some help from a young Illinois lawyer named Abraham Lincoln. The bridge went on linking the railroads for another decade, though with a few upgrades to handle damage from ice and larger railcars, and one replacement swing span , and a single pillar remains on the Davenport side to mark its place in history.

By 1872, the first bridge was inadequate to the task, and the first Government Bridge was built. Linking Davenport with the Rock Island Arsenal on its namesake island in the middle of the river, it was a double-deck iron structure set a couple hundred yards downstream of the original. The upper deck carried the railway, and the lower deck allowed pedestrians to cross between the two sides. Two decades later, this second bridge reached the end of its useful life when severely damaged by ice in 1896. This time, it was replaced by an all-steel, double deck, double track Pratt truss, set on the same piers as the destroyed 1972 bridge. This 1896 structure is the same Government Bridge that still carries IAIS and the highway over the waters today, though upgraded through the years to carry heavier traffic and, in the case of IAIS, doublestacks.

The line eventually became part of the great Chicago, Rock Island & Pacific, being double-tracked for large segments and handling a fair amount of the Rock's core freight. However, by 1975, the Rock was ailing, and then by 1980 the company was deemed unsalvagable and ordered liquidated by the bankruptcy courts. Between 1980 and 1984, the route was a bit chaotic, with small segments operated by a multitude of carriers. By mid-1984, the line was sold by the bankruptcy trustees to Heartland Corp., who founded Iowa Interstate as the operator for the entire Council Bluffs-Bureau line.