one-track mind, to have a

To be consistently fixated on or preoccupied by a particular topic. Jerry, you have such a one-track mind—not everything has to do with sex. My sister has a one-track mind, only thinking about how something can further her career.

Fig. to think entirely or almost entirely about one subject. Adolescent boys often have one-track minds. All they're interested in is the opposite sex. Bob has a one-track mind. He can only talk about football.

If someone has a one-track mind, they always think or talk about one subject. He has a one-track mind when he's working on a project — he thinks of nothing else. No, Sam, I wasn't referring to sex. You've a one-track mind, that's your trouble! Note: This expression is often used in a humorous way to say that someone thinks or talks a lot about sex. Note: This probably refers to a single-track railway, which will only allow trains to move in a single direction.

To be focused on a single issue, action, or undertaking; to be unable to take on more than one thing at a time. This cliché refers to the single-track railway line, which allows traffic in only one direction at a time. In a 1917 speech President Woodrow Wilson said, “I have a single-track mind,” one of the early figurative uses of the term.