This LIRR track map had an interesting detail ascribed to the Sea Beach line. Glancing at the map, the express tracks appeared to continue west for some distance, and the northbound local track had a short connection to a factory.
http://www.lirrhistory.com/emery/bayridge/br1751.jpg
According to The Brooklyn Daily Eagle, the Sea Beach right-of-way once stretched to the ocean, with the other end of the line ending somewhere around West 10 Street in Coney Island at the site of the Sea Beach Palace (for which it's named). The portion from the ocean to 3 Avenue was closed after electrification.