Having a one track mind refers to the instance that there is currently only one specific thing on your mind (which you may or may not make clear to your environment by returning again and again to said thing in conversations).
Guy: Dude, Barack Obama's speech was wicked last night!
The Other Guy: Uhmm, yeah, it was all right.
Guy: Oh, and dude! Did you hear about the puppy he wants to get? OMG
, it's so cool.
The Other Guy: Yeahh...
Guy: Hey, you know what dude? Barack Obama, ---
The Other Guy: Bro, you got one sick one track mind!
Guy: Let's just come back to Barack Obama for a sec...