What is the origin of the idiom "train of thought" and when to use it?

The idiom "train of thought" originated from the idea that thoughts can be linked together like a series of train cars connected on a track. It was first used in the early 16th century and has since become a common metaphor for the flow of one's ideas or mental processes. This expression is used to describe the sequence of connected thoughts or ideas that run through a person's mind as they think about a particular topic or problem.