Def.: The sensation of being so involved with something that it becomes all you can recreationally think about. Does not have to, and often does not, affect your regular work and responsibilities, but does take up all of your spare time.
Usage: 1) I swear once Halloween is over, I will have more interesting things to post on Sundays, but I have been in a state of productively one-track mind all month and since weekends are the time to think about it, between this state and actually being productive with schoolwork and whatnot also, I often am creatively worn out by the time they are over. 2) Productively one-track minds yield the most amazing end results that have ever been.
See also: creative exhaustion (n.)
–your fangirl heroine.