Apparently they would not be Metrolink trams, because the lines would still be used by normal trains as well, and there are obscure technicalities to do with signalling and power supply which mean that Metrolink can't share track wth normal trains. So they would be these new hybrid 'tram-trains', which already exist in Germany, it seems.
Sadly the prospect of Network Rail building completely new track is remote, to say the least!