Trams belong in the street, not in tunnels. When the tram is in the street, it is part of that street, part of the street furniture, part of the city. It's a facility. Trams shouldn't be thought of as a poor man's underground, an 'Underground' is something else.
In the street you can see a tram coming, and when on board and moving the city-scape slides past - all visual clues and familiarity. You are still 'there' - down town. A tram comes, you get on from the sidewalk, you get off straight onto the sidewalk.
But when trams are in tunnels you first have look for and find a 'station', go into 'The System', down the steps, into a different environment, and there you might become dis-oriented because the visual cues are gone. Then, at last you can get on board a tram. It sets off - into a tunnel. There is no scenery, just the tunnel wall - all those pipes! then, after a while, oops - this is my stop - I think.
Trams are not underground trains - they are a different thing. The idea that tram networks have to be changed into underground networks is a fundamental mistake. Your town will be the worse for it. Don't do it!