At left is a southbound N train on the Sea Beach Line about to pass beneath 19th Avenue. At right, heading south on the northbound-side express (inner) track (as you can see, the express track on the southbound side is no longer usable here), are a bunch of empty new R188 cars, eventually destined to be used on the 7 train, that are likely on their way down to the Coney Island Complex. (It's not the first time such cars have been spotted in the area.)
The Sea Beach express tracks have not been used for regular passenger service in quite some time. All of the station platforms along the line are on the outside of the tracks, so an express train would have nowhere to stop to pick up and drop off riders. When the express tracks were used for passengers decades ago, they provided nonstop service between 59th Street (on the 4th Avenue Line) and the terminal station at Coney Island.