From the February 1948 Official Guide:
Cincinnati Union Terminal hosted these railroads for passenger service:
Baltimore & Ohio
Chesapeake & Ohio
New York Central System
Louisville & Nashville
Norfolk & Western
Pennsylvania
Southern Railway System (CNO&TP)
It also hosted the Erie at its Freight House.
B&O: National Limited, Diplomat, Cincinnatian, unnamed 11/12 from Baltimore/Washington, 33/34, Washington Express from Pittsburgh, four numbered trains to Louisville, three to Detroit. The Cincinnatian name was soon moved to one of the Detroit trains.
C&O: FFV, George Washington, Sportsman from Washington. 19/20 Hammond IN (Chicago via Erie)
NYCS: Royal Palm, Ponce de Leon, Florida Sunbeam, six pairs of numbered trains to Cleveland, Ohio State Limited, Cincinnati Special, Carolina Special, plus various locals
L&N: Azalean, Humming Bird, Pan American, 3/4, 7/8. 17/18,29/30 to Winchester.
N&W: Cavalier, Pocahantas, Powhatan Arrow, 23/24
PRR: Cincinnati Limited, 213,267,227,202,206,222. From Chicago: Southland, Union, Cincinnati Daylight Express, Cincinnati Night Express
Southern: Royal Palm, Ponce de Leon, Florida Sunbeam, Carolina Special, Skyland Special
These are mostly from the condensed schedules. Many of the trains carried Pullmans that ended up on connecting trains, or which made across-the-platform connections for other destinations. I’m sure I missed a couple but the list illustrates how many trains called there on a given day.