This is the front side and main entrance to the St. Louis Union Station in 1950, and was the Southern terminus for the C&EI's Zipper.
(Unknown photographer, University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Museum of the American Railroad.)
ZIPPER
The Zipper was a named passenger train of the Chicago and Eastern Illinois Railroad. As No. 21 and No. 22, the Zipper served the Chicago, Illinois to St. Louis, Missouri route. The first incarnation ran in 1931 and was discontinued the same year. The second version of the train ran from 1937 until 1946. The Zipper connected with the Egyptian Zipper, trains 121 and 122, which traveled Danville to Cypress, IL, at Villa Grove, IL. The Zipper made the Chicago to St. Louis connection in 5 hours.
