The “train of thought” is a set of semi-related topics being mentally traversed or travelled down. Compare with the concept of the “wagon train”. The train of thought is a more structured form of “brain storming”.
Perhaps this idea is best described by discussing the concept of the “derailed train of thought”.. often this happens when a person comes up with all sorts of interesting related topics for a string of conversation and forgets why they were discussing that extra branch of topics in the first place.
“… and that’s why the English military had red uniforms. Why did I bring that up? Oh yeah, blood being red because of the iron. Hey, did you know that’s why Mars is red? Well it’s actually oxidized iron, and no there’s no oxygen in the atmosphere of Mars anymore.. whoops, there I go again.”
Compare a train of thought derailment with a “thinko” (typo).
TakeDown.NET -> “Train-Of-Thought”