Fig. a mind that thinks 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.
A mind limited to only one line of thought or action, as in All you think about is sex-you have a one-track mind. This expression, alluding to a train that runs only on one track or in one direction, was first recorded in 1928.