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.
It would also be more or less impossible under current US regulations, but there's always hoping that that could be fixed.