From some quick research it can't be running Stateliner as that was run out of Kedzie in 1961, CTA records from 8/61 show this bus being assigned to Lawndale at the time. Interesting enough this is the first time the bus is shown on a roster; this is a practically brand new bus. I checked what other routes Lawndale had at the time and none of them ran around Michigan/Chicago. The headsign doesn't give away much, every garages rollsign had a generic soldier field heading. My bet is some sort of CTA PR photo or fan trip, brand spanking new bus, no passengers, iconic place for a photo.
Sources I used: