Train of thought

The train of thought or track of thought refers to the interconnection in the sequence of ideas expressed during a connected discourse or thought, as well as the sequence itself, especially in discussion how this sequence leads from one idea to another.

When a reader or listener "loses the train of thought" (i.e., loses the relation between consecutive sentences or phrases, or the relation between non-verbal concepts in an argument or presentation), comprehension is lost of the expressed or unexpressed thought.[1]

The term "train of thoughts" was introduced and elaborated as early as in 1651 by Thomas Hobbes in his Leviathan, though with a somewhat different meaning (similar to the meaning used by the British associationists):

By Consequence, or train of thoughts, I understand that succession of one thought to another which is called, to distinguish it from discourse in words, mental discourse.
When a man thinketh on anything whatsoever, his next thought after is not altogether so casual as it seems to be. Not every thought to every thought succeeds indifferently.

— Thomas Hobbes, Leviathan, The First Part: Of Man, Chapter III: Of the Consequence or Train of Imagination