Judging by the following newspaper article, We can assume that passenger service to the Sterling C.B.&Q. Depot ended about 1942.

Old Passenger Depot Sign is painted over
Tuesday, May 19, 1942
From the Daily Gazette

The plain large white sign with its bold black letters painted several score years ago which is over the main entrance to the C.B.&Q. station in Sterling has been painted over and soon a new set of letters will greet the passerby. The exterior of the station as well as the interior is being painted anew.
This old sign was a reminder to many of the early days when the ‘Q’ first entered Sterling. It would be interesting to know how many thousands of train passengers were informed that this was Sterling. Among them was President Woodrow Wilson. The sign is such as motion picture show patrons would see of stations in Wyoming, plain and substantial, no ornamentation. It just stated ‘Sterling’.