Right. So thru the miracle of cut-and-paste, assume the trigger "zone" is as depicted above in the orange box and "created" using insulated fibre pins or the equivalent to isolate a section of outer rail.
When any metal train wheel/axle straddles the outer rails, this electrically applies the U post to the insulated rail trigger section. This then applies U-B (which was set to fixed 14V) to the banjo. And it doesn't matter what the train throttle (U-A) was at the time...it could be 6V up to 20V. The banjo will only receive the fixed 14V.
