Couple points. First, it’s the conductor, not the motorman who gets shot dead in the back (though it was the motorman in the original novel). Also, the gag involving Garber’s reaction to the fact that the Inspector is black was a remnant of the fact that in the original novel and the first draft of the script Garber’s character was a black cop named Presscott. The gag originally played with the black Presscott going to meet the white Daniels and Daniels had the line when Presscott arrives. When they couldn’t get a star black actor for the role they changed black Lieutenant Presscott to Jewish Lieutenant Garber and then made the Daniels character black.