Scioly.org

The rules talk about a POI (point of intersection)
Roller Coaster
3k. Loop: The device may contain one loop in the track to earn a Loop Score. A loop is defined as a continuously concave section of track which appears to self-intersect at a Point of Intersection POI.

Is this a physical POI or can the POI be a helical loop that crosses over itself???

The POI is defined as the point where the entry to the loop and the exit of the loops appear to self-intersect. Just view the marble from the side, and it's the point where the two parts of the track appear to intersect.

Note that the rules require that the tracks touch at the POI. This makes it hard to use the "two rails" style track for a loop, because you'd expect the edge of the marble to extend beyond the "rail", and there's another track there...