Another issue is that systems might cross several tiers: Southeastern
I like the solution that was implemented for Boston where the Green Line light rail and Mattapan trolley are broken into a separate system from the heavy rail subway lines
And even routes - eg The S1 in Karlsruhe - running as on-street tram, semi-segregated light rail, underground metro, and regional rail at various points along the route!
That's a light rail (tram) line, based on photos of it. Belongs in tier 4.
I'm not an expert for railways. Thus, I don't get what's the different for your tier 3/4 suggestions.
There are multiple distinguishing features, but heavy rail rapid transit systems:
- always run on their own ROW, not in streets
- have high level floors and are typically boarded from high level platforms
- typically involve longer, higher capacity trains
Light rail systems meanwhile:
- run in a mix of their own ROW and in streets
- typically have lower floors and lower platforms
- typically involve shorter, lower capacity trains
These distinctions also have some functional implications because light rail trains can generally handle steeper hills and navigate tighter curves than their heavy rail counterparts, and also have shorter stopping distances (they have to, to safely run in streets).
"Light rail" is also admittedly an American term, but British English "tram" is used similarly.
Good rule of thumb is if it looks more like
this it's light rail, but if it looks more like
this it's heavy rail.