It doesn't because it doesn't use any mainline rail track. Imagine if the T-Third went onto the Caltrain tracks at Bayshore and continued down the peninsula instead of terminating at Sunnydale; that would be a tram-train.
It would also be more or less impossible under current US regulations, but there's always hoping that that could be fixed.
Tram trains are mostly meant to give lightly used train lines a new lease of life with a substantially more convenient urban alignment, since for historical reasons legacy rail lines tend to skirt around downtowns and avoid where most of the jobs and people are.
That is not just a capacity and speed issue, it is also a safety issue. You don't put "light rail" and "heavy rail" on the same track, because a collision will be catastrophic to the light rail.
You can in Europe (and now in the US since 2018) because realistically, everybody is screwed no matter what kind of high speed collision happens, and in both regulatory frameworks it is substantially safer to simply have signalling and automatic emergency stops to prevent collisions in the first place (ETCS in Europe and PTC in the United States), and then to outfit train cars with better safety technologies like anticlimbers and crumple zones.
We stopped requiring buff strength in automobiles as the only thing a long time ago because it turns out that mostly just resulted in the cars surviving and the people inside them dying. Try throwing a steel box full of eggs and see what happens to the eggs.
