Brookfield Properties has hired Skidmore, Owings & Merrill to design two office towers that will rise on a platform over railroad tracks near Ninth Avenue. Work is slated to start in June on the towers, which will be 1.9 million square feet and 3.4 million square feet. Brookfield hopes to sign an anchor tenant during the two-year construction project. A rendering of the project appeared on the Observer’s Web site today. Brookfield also hired Skidmore to design its Hudson Yards proposal.