London overground is a frequent metro rail service using proper trains, no trams or street running, yes its a great success but were not being offered that.
London Overground was a run down delipidated, unloved service with a low frequency. I know. I used to use it. It was horrendous. Now it is excellent. That's the point.
Frankly the type of vehicle, and whether it's Street running or not. It's about the level of service. High frequency, reliability and modern vehicles get people to use services.
The original Metrolink's lines were heavy rail. Within months of conversion they accounted for more than half of all rail usage in Manchester.
Still not sold? Head to London again. The core of the London Tramlink network was a series of unloved, barely used railway lines going "nowhere useful". I used to live near the Wimbledon line. That line now runs every few minutes until 1am. The system has plenty of street running. It hasn't held it back.
Trams Vs trains. Heavy Vs light. Street Vs off street. It's irrelevant. It is all about the service. Nothing else matters. Get the service right and it will work.