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.
Words nearby one-track mind
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.