From Longman Dictionary of Contemporary Englishone-track mindˌone-track ˈmind noun → have a one-track mind
Examples from the Corpus
one-track mind• Now, an animal has a one-track mind.• I was a fully equipped birdwatcher, very inexperienced, perhaps, but with a one-track mind.• But all my wit and wisdom were wasted on Karen's one-track mind. 