Longman Dictionary of Contemporary English
one-track mind
noun
EXAMPLES FROM CORPUS
▪ But all my wit and wisdom were wasted on Karen's one-track mind.
▪ I was a fully equipped birdwatcher, very inexperienced, perhaps, but with a one-track mind.
▪ Now, an animal has a one-track mind.
Wiktionary
one-track mind
n. (context idiomatic English) That said to be possessed by someone who is obsessed with something or only able to think of one thing.
Usage examples of "one-track mind".
At this range, even a Sidewinder might scoot past the target before its one-track mind could track on the MiG's exhaust and correct the missile's course, and if he dropped back for more room, the more maneuverable Fulcrum would give him the slip.
Cut-and-dried ideas, a one-track mind, harsh principles, and integrity, absolute integrity….
Pike Purcell had a one-track mind and that one track was busy with surmises over the story told him by Dud.