The phrase “train of thought” dates back to the 1600s, well before locomotives existed. It comes from an older meaning of the word “train” that had nothing to do with railroads. The philosopher Thomas Hobbes used “Trayne of Thoughts” in his 1651 work Leviathan, making it one of the earliest known appearances of the phrase in English.
The Original Meaning of “Train”
The word “train” has been part of English since the 14th century. It comes from the Anglo-French word trainer, meaning “to draw” or “to drag,” and it originally referred to the part of a gown that trails behind the wearer. Think of a wedding dress with fabric flowing along the floor. That trailing fabric was the “train.”
From there, the word expanded naturally. A group of attendants following behind an important person became a “train” of followers, like a human version of the trailing gown. Then it stretched further to describe any moving line of people, animals, or vehicles. A line of black powder leading to an explosive was also called a train. In every case, the core idea was the same: things connected in sequence, one following another. When locomotives came along in the 19th century, a series of connected cars pulling along a track fit the pattern perfectly, and the word attached itself to yet another meaning.
So when someone in the 1600s spoke of a “train of thought,” they meant a sequence of ideas trailing one after another, each one dragging the next along behind it.
Hobbes and the Philosophers Who Built the Metaphor
Thomas Hobbes gave the phrase its first major intellectual framing. In Leviathan (1651), he defined it precisely: “By Consequence, or Trayne of Thoughts, I understand that succession of one Thought to another, which is called (to distinguish it from Discourse in words) Mentall Discourse.” For Hobbes, a train of thoughts was the mind’s silent inner movement from one idea to the next.
He also gave a vivid example of how these mental chains work. During a conversation about the English Civil War, someone suddenly asked about the value of a Roman penny. The question seemed absurd and unrelated. But Hobbes traced the hidden links: the war brought to mind the king being handed over to enemies, which called up the image of Christ being delivered to his captors, which triggered the memory of the 30 pieces of silver paid for that betrayal, which led naturally to a question about Roman currency. “And all this in a moment of time,” Hobbes wrote, “for thought is quick.”
The slightly more formal version of the exact phrase, “train of thought,” appears in a 1688 work by Roger L’Estrange, where he wrote: “In a Train of Thought it comes Naturally now to be Enquir’d into…” By that point the expression was settling into common English usage.
Why Philosophers Were So Interested
The phrase caught on partly because 17th and 18th century thinkers were deeply occupied with a specific question: why does one thought lead to another? John Locke coined the term “association of ideas” in 1700, and the British empiricist philosophers who followed him turned the concept into a full theory of how the mind works. In their view, ideas were like mental images, and they linked together through patterns of association. If two things had appeared together in your experience before (a smell and a place, a word and a feeling), encountering one would pull the other into consciousness.
David Hume described it this way: “There is a principle of connexion between the different thoughts or ideas of the mind, and that, in their appearance to the memory or imagination, they introduce each other with a certain degree of method and regularity.” The metaphor of a train, with each car coupled to the next, fit this perfectly. Your thoughts weren’t random. They followed a track, pulled along by association.
James Mill later pushed this even further, arguing that consciousness, memory, and imagination were all just different segments of the same ongoing train of ideas. The phrase wasn’t merely a figure of speech to these thinkers. It was their working model of the mind itself.
How “Train of Thought” Differs From “Stream of Consciousness”
By the late 1800s, a competing metaphor appeared. The psychologist William James proposed calling mental life “the stream of thought, consciousness, or subjective life.” Where a train implies orderly, linked cars moving along a fixed track, a stream suggests something fluid, continuous, and harder to pin down.
The difference matters. A train of thought is structured. It moves from point A to point B through a sequence you can retrace, like Hobbes did with his Roman penny example. A stream of consciousness, especially as it came to be used in literature, captures something messier: the full rush of impressions, memories, and sensations flowing through awareness before they get organized into tidy logical sequences. Writers who adopted stream-of-consciousness technique used long unpunctuated sentences, jumbled impressions, and fragmented grammar to capture that raw mental flow.
Both phrases survive because they describe different aspects of thinking. When you carefully reason through a problem, you’re following a train of thought. When your mind drifts through loosely connected images while staring out a window, that feels more like a stream.
What Happens When the Train Derails
The phrase also lives on because “losing your train of thought” is such a universal experience. Modern cognitive psychology has studied exactly this. Research on working memory and attention shows that maintaining a train of thought is an active process requiring what scientists call executive control: the ability to keep your current goal in mind and resist distraction.
People with higher working memory capacity are better at staying on track during demanding tasks. Those with lower capacity tend to experience more mind wandering, where task-unrelated thoughts intrude and pull attention away from whatever they were doing. Interestingly, this mind wandering doesn’t seem to require mental effort. It appears to be what happens by default when the executive control system momentarily drops its grip on the current goal. Your train of thought doesn’t just stop. It gets hijacked by a different train.
So the 17th century metaphor holds up surprisingly well. Hobbes described thought as one idea dragging the next behind it through chains of association. Modern psychology confirms that maintaining a coherent sequence of thought takes real cognitive work, and that the links between ideas can easily pull you somewhere you didn’t intend to go.