John is alternately wearing officer's collar brass, then enlisted man's collar brass in the same scene. He has the rank of sergeant on his sleeve, but the girl in the train station addresses him as "lieutenant."
Aside from the fact that the main character in the movie is referred to as a lieutenant multiple times, but wears the stripes of a Sergeant First Class, his uniform jacket also displays a "ruptured duck" patch on his right side. This patch is what was given to military personnel at the end of World War 2, when they were discharged from active duty. In this story, he has just finished preliminary training.
John is shown just out of Army basic training with the rank of Sargeant First Class. It is very unlikely that a soldier would emerge from basic with such an advanced non-commissioned rank.
In several places the movie refers to an address on West 46th Street. In Philadelphia the numbered streets run North and South.