At least four high-rise towers are planned at Miami’s All Aboard Florida rail hub. Renderings released Wednesday show Florida East Coast Industries LLC expects to develop 3 million square feet of office, retail and residential space on nine acres between Northwest Second and Eighth streets west of Northwest First Avenue. Building heights were not disclosed, but a drawing shows the southernmost building towering over the 31-story Stephen P. Clark Government Center to the south. A 50-foot-tall station platform would be the southern end of Miami-Orlando passenger rail service.