one-track mind 
noun
Used in reference to a person whose thoughts are preoccupied with one subject or interest.The ones who are guiding the government's economic decisions from inside or outside - mostly former World Bank and IMF employees - have one-track minds....
- . Having decided that he's going to ride the sponsorship scandal to the finish line, he had a one-track mind in these debates.
- The tram ride all the way to Spencer St Station is futile, with my sleep-deprived one-track mind hallucinating so there is a giant Red Bull can on top of every blue sedan I see.