A train of thought - Idioms by The Free Dictionary
a train of thought
train of thought
One's uninterrupted progression of thinking. When the phone rang, it derailed my train of thought—I totally forget what I was talking about! Please don't interrupt my train of thought when I'm writing.
a train of ˈthought the connected series of thoughts that are in your head at a particular time: The phone ringing interrupted my train of thought.