train of thought vs thought

train of thought and thought both are nouns.

Word	Noun	Adjective	Verb	Adverb
train of thought	Yes	No	No	No
thought	Yes	No	No	No

As nouns, thought is a hypernym of train of thought; that is, thought is a word with a broader meaning than train of thought:

train of thought: the connections that link the various parts of an event or argument together
thought: the process of using your mind to consider something carefully

Other hypernyms of train of thought include cerebration, intellection, mentation, thinking, thought process.

train of thought (noun)	thought (noun)
the connections that link the various parts of an event or argument together	the process of using your mind to consider something carefully
	the content of cognition; the main thing you are thinking about
	the organized beliefs of a period or group or individual
	a personal belief or judgment that is not founded on proof or certainty