I think that gets into whose definitions you believe. IIRC, the Federal Transit Administration divides things into commuter rail (anything that has ordinary track connections that can be used to move a whole train at once onto the tracks of a traditional freight railroad will be in this category), heavy rail, and light rail, as Alan describes. I believe the Federal Railroad Administration defines heavy rail as anything that interconnects with the freight railroads and therefore falls under the jurisdiction of their rules, and light rail as the various forms of isolated passenger only tracks.
I don't think that in
FRA terminology, what is call
heavy rail generally falls under FRA jurisdiction. Things like NYC Subway, BART, Boston Subway (Red, Orange, Blue) are
heavy rail and they are all
FTA and not
FRA jurisdiction, and they may only have a notional connection to a regular railway line, but typically don't anymore. There are some
heavy rail lines like PATH which for historical reasons are under
FRA jurisdiction but operate mostly under waivers, and even PATH does not really have a real connection to regular railways anymore.
OTOH,
FRA jurisdiction lines sometimes have
light rail service, e.g.
RiverLINE in NJ. At present this is done using a technique called
temporal separation. However the trend seems to be in a direction wherein eventually intermingling of light rail and commuter/freight rail will be allowed (as is general practice in Europe and Japan) provided the signaling and control system provides a feature called
positive train separation. In this context the term
light rail does not include what is traditionally called
tram in Europe. It involves only the heavier variety of
light rail exemplified by equipment like the
Stadler cars used on the
RiverLine.
BTW,
FTA until now does not have any uniform safety regulation that they enforce. They are in the process of crafting such, much to the consternation of some. The last round of WMATA accidents which were traced to what can charitably called gross negligence is what gave the impetus for this.
