This line started out as the Saint Louis, Kansas City and Colorado Railroad, who built the first vestige of the line between Saint Louis and Union in 1887. The intent was to build a line into Colorado via Kansas City. By 1901, the StLKC&C had further completed their line through Gerald and Owensville to Bland.
Meanwhile, the Chicago, Rock Island and Pacific Railroad was interested in extending their empire westward from Saint Louis to Kansas City, and watched the development of the StLKC&C with a keen interest. They purchased the StLKC&C ourtight in 1902, and continued extending the line into Eldon (which would eventually become the subdivision's base of operations, complete with a roundhouse and large yard), Versailles, and Windsor (and a connection with the Missouri-Kansas-Texas Railroad there) in 1904. Trackage rights were utilized over the M-K-T to allow the Rock Island access into Kansas City from Windsor; the Rock Island finally completed their own line into Kansas City (via Pleasant Hill, Lee's Summit and Raytown) in 1905. The line was known as the Rock Island's Saint Louis Subdivision.
Despite connecting two of the nation's largest railroad-centric cities, the Rock Island's line failed to meet expected revenue, and ultimately was downgraded into secondary status. The Rock Island was a late-comer to the Saint Louis-Kansas City corridor, which had already been previously connected by four other railroads, so the Rock struggled to find its piece of the pie in a saturated market. Moreover, the line itself was costly to maintain, with a number of tunnels and high bridges required to cross Missouri's rugged terrain in and around the Ozark Mountains region.
Passenger service, which at the end consisted of only two twice-daily motor cars, called "doodlebugs" (each of which left Kansas City and Saint Louis, respectively, and met in Eldon), ceased on April 11, 1959. As the financial woes of the Rock Island continued on their downward pace, maintenance of the line suffered.
The Rock Island entered its 3rd and final bankruptcy in 1975. In an effort to continue service along the route, the Interstate Commerce Commission directed the Kansas City Terminal Railway to oversee the line's operation while the Rock Island was under an attempted reorganization. When it was decided the Rock Island could not be reorganized successfully, the Kansas City Terminal Railway continued their operations indefinitely, until the Saint Louis-Southwestern Railroad, a.k.a the Cotton Belt Railroad, took over. However, before long, the Cotton Belt diverted all of their traffic to the former Missouri Pacific mainline, acquired as part of the UP-MP merger. The Cotton Belt continued local service over the eastern portion of the ex-Rock Island line, with the western portion (between Belle and Kansas City) laying unused. Traffic was scaled back to Owensville in the mid-1980s, with additional scaling back to Union in 1995. Even so, the majority of traffic on the line occurred in Saint Louis proper, with only a few trains sent out to Union on an as-needed basis. When subsequent owner Union Pacific attempted to abandon the line west out of Saint Louis, local opposition resisted, saving the line from the chopping block.
Today, some of the right-of-way is owned by a local coal company, which is served by a parallel Union Pacific route; the corridor lays dormant, awaiting a potential decision of the coal company to institute their own service over the line. In addition, the portion of the route west out of Saint Louis into Union is now served by the Midland Central Railroad. Otherwise, the remainder of the former corridor remains, either abandoned or out-of-service, in what is probably one of the longest continuous disused rights-of-way in the country.
Thanks to Brian Contestabile for contributing information about this route.