How did Einstein use the conclusions of special relativity to reach the equivalence principle (using the Elevator thought experiment)?

Question

Could someone please explain how Einstein used the Elevator thought experiment and the principles of special relativity to conclude that uniform accelerates creates uniform gravitational field (equivalence principle)?

Answer 1

He did not derive it from special relativity. He thought about it and decided it must be true. He postulated it. He called that realization "the happiest thought of my life" and it would guide him toward the development of General Relativity.