The problem is that there aren't clear dividing lines. What is the BART in the Bay Area? I would say more commuter rail, but I believe the Feds classify it as heavy rail. Functionally, it's clearly commuter rail.
Obviously some systems are clearer. I don't think anyone would call the NYC subway a light rail line, or the San Diego trolley a subway, but there are many grey areas. In Europe, a "Metro" can be heavy rail or light rail. Most of the German U-Bahn systems are actually light rail, though they run underground in parts, or some, like Frankfurt are hybrid systems.
Similarly, most of the Metros they're building in China are actually hybrid heavy/light rail networks.
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