Sentence:  A man may be absorbed in the deepest thought, and his brow will remain smooth until he encounters some obstacle in his <WSD>train</WSD> of reasoning, or is interrupted by some disturbance, and then a frown passes like a shadow over his brow. 
Meaning of the word marked with <WSD> tags:  A sequence of events or ideas which are interconnected; a course or procedure of something. (from 15th c.)