Underdog Stories / Everyday expressions
What Does “Train of Thought” Mean, and Where Did It Come From?
What does train of thought mean and where did it come from? Trace Hobbes’s 1651 wording to a sense of train that long predates the railway.
By Brett Hanson · Underdog Podcasts · September 9, 2026
Twenty has a narrow use for the phrase. On Appraise The Phrase, in the episode “Train Of Thought,” the co-host tells Mario Hines he almost always says it after losing one. Often, he said, he wants to recover the last word he was saying.
A train of thought is a sequence of connected ideas: one thought brings on the next. If you lose yours, you can no longer follow the line you were taking. The expression appears in Thomas Hobbes’s 1651 book Leviathan. Its “train” comes from an older word for things following one another, long before railway trains had that name.
What does train of thought mean and where did it come from?
The key is the link between thoughts. You think about a pencil on your desk. That reminds you of a note you meant to write. The note brings to mind the person waiting for it. Each thought leads to another, even if you never sat down to reason anything out.
Mario brings several dictionary meanings to the episode. Cambridge describes connected thoughts; another definition calls them a path of reasoning. He doubts that every train needs a goal. Twenty agrees. A ringing phone could make him forget what he was saying about a pencil. Nobody needs a grand theory of pencils for that to happen.
They have found a useful gap between the formal meaning and daily use. A train of thought may be a careful argument, yet it can also be a loose run of ideas. What makes it a train is their succession. One thought follows another closely enough that you could, at least for a moment, retrace the route.
That also explains why people often notice the phrase only when the route breaks. Mario said that when a train of thought is working, “you won’t pay it no mind.” Twenty’s lost last word may sound smaller than a whole series of ideas. In conversation, though, that word can be the clue that brings the rest back.
Where did “train of thought” first appear?
Thomas Hobbes put the expression in Leviathan, published in 1651. Chapter Three deals with what he calls the “Trayne of Imaginations.” He describes a “Trayne of Thoughts” as one thought succeeding another, then gives that silent activity another name: “Mentall Discourse.”
The old spelling is worth a glance. So is the plural. When Mario reads the passage aloud, Twenty catches the word thoughts at once: “He put the S.” Earlier, the two had teased out whether “train of thoughts” sounded natural. Hobbes, inconveniently for their ears, had written just that.
Finding the phrase in Hobbes does not prove he invented it. It proves less. A surviving printed use shows that someone wrote the words by that date, though it cannot tell us who first said them across a table. Hobbes offers more than a bare sighting: he stops to explain what he means.
He also gives a wonderfully odd example. In a talk about the English Civil War, someone asks about the value of a Roman penny. Hobbes follows the jumps in between, from the war to the king being handed over, then to Christ’s betrayal and its thirty pieces of silver. To anyone hearing only the question, the penny seems to have arrived from nowhere.
Hobbes thought the links happened quickly. His chapter leaves room for thoughts that wander as well as thoughts aimed at a purpose. The Roman penny is his way of showing how a mind can travel a considerable distance before anyone else sees it move.
What did “train” mean before railroads?
Picture the back of a long gown trailing across a floor. That was one use of train in English by the late 1300s. The word came through Old French, where it had to do with a trail or the act of dragging something. It could also refer to people following in a procession.
From there, train could mean a succession, one thing following another. No wheels required. You can see how that meaning fits Hobbes’s thoughts without putting a single wheel on a track, and Etymonline records train meaning a series or continuous course by the late 1400s.
The railway meaning came later. Etymonline records train for a locomotive with coupled cars in a work published in 1820 and dated 1816. Even using the earlier date, that is 165 years after Leviathan. The dates make the tempting railway explanation impossible for Hobbes’s passage.
“He put the S.”
Mario admits that a railway train is the picture the phrase gives him now. That is easy to understand. The cars follow one another, and a train of thought does much the same in the mind. The image works; its timeline doesn’t. The older word helped name the vehicle, rather than the vehicle giving Hobbes his phrase.
There is a smaller surprise here, too. Hobbes was writing about thoughts following one another before the familiar railway picture existed to help a reader along. He trusted the plain idea of a train, something drawn behind or moving in succession, to do the job.
Did Hobbes mean careful reasoning?
Sometimes. Hobbes divides his mental trains into two kinds. One wanders without a set aim, the way thoughts can in a dream. The other is guided by something a person wants or fears. In that case, one thought suggests a way forward, which suggests another.
That makes the episode’s argument over “path of reasoning” more interesting. Mario resists the idea that every train of thought must be trying to reach an answer. Hobbes’s own chapter supports that doubt. His wandering train still counts, even when its links look strange to someone standing outside his head.
Take the Roman penny. The route makes sense once Hobbes lays it out. Yet the speaker had apparently crossed those links in a flash, without presenting them as a step-by-step case. A train of thought can have a route without being a plan.
The difference matters when someone says, “Follow my train of thought.” They may be asking you to follow an argument. They may just be asking for patience while they explain how they got from a civil war to pocket change. Hobbes left space for both sorts of travel.
What did John Locke add to the idea?
John Locke later wrote about the habits that join ideas in the mind. The dates matter. His Essay Concerning Human Understanding first appeared with a 1690 date on its title page, while the chapter called “Of the Association of Ideas” arrived in the fourth edition in 1700. Give the whole book one date and that distinction gets blurred.
Locke describes thoughts that fall into familiar patterns. One of his examples is a musician who starts a tune: the notes follow in the musician’s mind while the fingers move over an organ’s keys. Locke uses a train of motions to describe how a habit can keep going once set in motion.
That example feels less remote than the phrase “association of ideas” might suggest. Hear the first bit of a song you know well, and the next bit may arrive before you ask for it. Locke’s point is about how often-used links become easy to follow. He even pictures a path worn smooth by repeated steps.
The hosts have a version of that problem in ordinary speech. Twenty reaches for the familiar phrase “lost my train of thought” without needing to picture a long series each time. Then the phrase itself makes him stop and ask what, exactly, he has lost.
Why do we say “I lost my train of thought”?
Because an interruption can leave you unable to pick up the connection you were following. You still know plenty of things. You may even remember the subject. What has gone missing is the next step, or the small clue that would lead you back to it.
Twenty said he might need only the last word. Mario pictured more of a series. Both descriptions fit what happens in conversation: the word can recover the thread, while the thread can recover the point. A phone rings, someone speaks, and the next link fails to show up on time.
The phrase also lets you tell other people why you have stopped mid-sentence. It asks for a moment without making a speech about memory. That is probably why the lost version comes so readily to Twenty. Most of us have had reason to say it, often while staring at the person who interrupted us.
Near the episode’s end, Twenty tries to gather his train of thought. Then he says he has nothing. Mario catches the moment: “Lost it.” Hobbes had the Roman penny. Twenty had a blank.
The full episode of Appraise The Phrase is on Underdog Podcasts, with the rest of the network's shows.
People also ask
Was “train of thought” named after a railway train?+
No. Thomas Hobbes used “Trayne of Thoughts” in *Leviathan* in 1651. The railway sense of *train* is recorded much later, in a work published in 1820 and dated 1816. Hobbes was using an older meaning tied to things following one another in succession.
Did Thomas Hobbes invent the phrase “train of thought”?+
Hobbes’s 1651 book gives us an early recorded use and an explanation of the phrase. That does not prove he was the first person to say it. Written records show when words appeared on a surviving page; they rarely tell us what was first said in conversation.
Is “train of thoughts” correct?+
It has a strong historical example. Hobbes wrote “Trayne of Thoughts,” with a plural, in *Leviathan*. Today, people commonly say “train of thought,” especially in “I lost my train of thought.” The older wording caught Twenty’s ear when Mario read it aloud in the episode.
Does a train of thought have to be logical?+
No. It needs a connection from one thought to the next, though that connection may be hard for someone else to see. Hobbes described both wandering thoughts and thoughts guided by a goal. Mario makes a similar point in the episode: thinking about a pencil need not be a formal argument.