Landmark has been short on specifics of its plan. But the chamber’s report said the development could bring almost 20 million square feet of skyscrapers, including homes, offices and hotels, to a 34-acre platform that would be built over train tracks between McCormick Place and the Field Museum. It could cost up to $19 billion, including $3.8 billion to build the transportation infrastructure, the report said, using information from the developer.