Maybe this revised diagram will clarify?
Electricity has no concept of left-to-right or right-to-left! Current flows where the wire takes it...up, down, left, right, north, south, or around and around! 
The 153IRs are NOT electrically connected to the track! They do physically sit next to the track at 3 places to detect trolley occupancy at those positions. Power to the 153IRs comes from the AC-to-DC converter output. The rocker switch on each 153IR is set to "AUX PWR" and the DC power is applied to the U and A terminals on the left side of each 153IR.
To be clear, your 20 foot run is a single "block" of track. The center-rail and outer-rails are continuous from end-to-end. There are no cuts or breaks or "isolated" sections. The timer module has a relay which has ON-OFF control to the entire 20 foot track. All or nothing!
I clumsily attempted to show 3 power drops along the 20 feet of track.
