The phrase "breaking train thought" implies that one's thoughts are interrupted or derailed. Antonyms to this phrase would include phrases that suggest continuity or uninterrupted thinking. Some possible alternatives might include "flowing stream of thought," "unbroken chain of thinking," or "cohesive train of thought." Each of these phrases suggests a sense of connectedness or coherency in one's thinking. Other possible antonyms might include phrases like "uninterrupted thinking," "unhindered thought process," or "undisturbed thought pattern." Whatever phrasing one chooses, the goal is to convey a sense of focus and clarity, free from the jarring interruptions implied by the phrase "breaking train thought.