A The answer is quite simple: At each location, the railroads have joined together to operate a single facility. Many large cities had such stations, such as Los Angeles, Denver, Kansas City, St. Louis, Chicago, Nashville, and Jacksonville.
Chicago Union Station, for example, was a joint venture of the Chicago, Burlington & Quincy; Milwaukee Road; and Pennsylvania Railroad through the jointly owned Chicago Union Station Co. (The Gulf, Mobile & Ohio, which also called at the facility, was a non-owner tenant.)
Smaller cities could also benefit from joint-use facilities, such as Effingham, Ill.; Marion, Ohio; and Bethlehem, Pa. Passengers were able to connect between multiple railroads’ passenger trains, without changing stations. – Brian Schmidt
