a focus on one issue or topic, to the exclusion of all else; an obsession with one thing
She just has a one-track mind, wanting to move on.
1938; fr. railways
"Thank God that thing had a one-track mind, and was concentrating on the spider," said Jim, with a rather humorless laugh.
Once his one-track mind got to functioning on a certain objective it seldom digressed.
A mind limited to or obsessed with a single idea, theme, etc (1927+)