Construction could start by early 2021 on the stalled Back Bay air-rights development — a 1.26-million-square-foot mixed-use project that includes offices, retail, two residential towers, with an expanded, renovated transit station — but developers said they hope to secure major tenants before their shovels go in the ground.

Boston Properties — the commercial real estate developer who is also behind the Hub on Causeway — two years ago earned approval from the Boston Planning & Redevelopment Agency to renovate a parking garage and construct a 26-story office building and two residential towers that would bring 600 new apartments over Back Bay Station. The project also includes $37 million to renovate and expand the station that serves 18,000 commuters a day and create a new station entrance on Stuart Street.