1
general::
Phrase(s): someone’s train of thought
Fig. someone’s pattern of thinking or sequence of ideas; what one was just thinking about. (See also lose one’s train of thought.) • My train of thought is probably not as clear as it should be. • I cannot seem to follow your train of thought on this matter. Will you explain it a little more carefully, please?
McGrawhill's American Idioms And Phrasal Verbs
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general::
A succession of connected ideas, a path of reasoning, as in You've interrupted my train of thought; now what was I saying? This idiom, which uses train in the sense of "an orderly sequence," was first recorded in 1651, in philosopher Thomas Hobbes's Leviathan.
American Heritage Idioms