Michigan Central (MC) was not happy with an eastern competitor joining IC and MC at the Illinois Central stations. (IC had more than one station in the 1800s as it kept outgrowing the last one it built.) So when IC elevated its tracks around the turn of the 19th Century, the B&O did not build a connecting ramp. Instead, they took advantage of the financial difficulties of the Chicago area properties built and acquired by the president of the Northern Pacific including Grand Central Station to form the B&O Chicago Terminal (C&OCT) Railroad in 1910.
Brandon McShane provided more history of their early depot usage in a comment on a posting:
B&O used a portion of the Inter-State Exposition Building, roughly on the site of the Art Institute, as its Chicago terminal until IC's track elevation through Hyde Park and Woodlawn broke the connection at Brookdale. The first B&O passenger trains called on Grand Central Station on Decembr 1, 1891.The Brookdale Spur served as an industrial spur until the 1970s. Forgotten Chicago documents the industries served and how the RoW has been reused.
While studying the 1897 Map, I noticed that this spur allowed the B&O, as well as the IC, to directly serve the 1893 Columbia Exhibition.
Update: see photos of the 76th Street Depot.
A photo of the IC south Branch crossing the Brookdale branch at Commercial and 83rd. In the comments I discuss this as evidence of the track density that used to be in Chicago. Thus the label "just horses".