"Lost steam" is an idiomatic phrase that refers to slowing down or losing momentum. The antonyms for "lost steam" could be "gaining momentum," "building speed," "accelerating," "picking up steam," or "gaining traction." These antonyms suggest a surge of energy, progress, or forward movement. They imply movement or progress towards a goal or objective. When something gains momentum, it means it is moving faster or becoming more powerful. Building steam suggests that progress is happening, and things are moving forward. Accelerating implies that something is moving faster and quicker. Picking up steam denotes that something is gaining more energy and drive towards its objective.