Miniatronics Automatic Reversing Unit

[Curt]

I just purchased a automatic reversing Unit (Miniatronics RU1-1). I am still a little bit of a beginner at model railroading but I am just wondering what rail is the little light sensor put on? I know there are two sensors that go at each end of the section where I want the train to reverse and go the other way, but what rail does that sensor go on. My current train is set up with blocks as far as current goes. I would appreciate any help that someone could give me? Thanks.