Not sure why regulations in North America are so different than in Europe, but in Europe subway cars can and do intermingle with (freight) rail (not in revenue service, to be sure, but when they need to be transported to/from the city they're being used in), but trams most definitely don't (some tram-trains can operate on a section of mainline track, but I've never seen / heard of a tram coupled in a freight consist, quite unlike subway cars), so at least in Europe it makes sense to call subways heavy rail and trams light rail in absolute terms, even though subway cars there aren't necessarily built more heavily than here. In any case, mainline rail is simply the next logical step above subways, the relative use of "light" vs "heavy" doesn't preclude the existence of an absolute hierarchy (mainline rail > subway > light metro > LRT).Okay, here is where it gets absolute.
The mainline railways - CN, CP, VIA, GO and the like - are all considered heavy railways. And there are legal and regulatory definitions that reinforce this.
And thus, anything rail-based that can't intermingle with them - subways, streetcars, etc. - are considered light rail.