The pearl handled pistol that Frank Olchin removes from his sock drawer was sitting on his desk in the study, the room he just left.
When Leo looks in the rear view mirror and sees Willie and Granada taking off their clothes, they are on the driver's side of Granada's car. When he turns around and looks, they are now on the right side of the vehicle.
The cars in the "subway yard" are not actually subway cars; they are circa-1963 suburban commuter coaches for the Metro-North & Long Island Railroads.