There's a lot of devils in the details here - but yes, as a generality, interurban-style light rail can be done on far lighter-duty rails (& bridges, etc.) than even the bottom tier of "regular" railroading requires.

Though it's not just the engines that are heavy on regular RR's. And many of the costs do not scale with track weight. And running a regional/intercity transit system is seriously non-trivial, even if Santa Claus somehow gifts you with a free and magically maintenance-free rail network.