Please come to Seattle and talk to the people who want to spend hundreds of millions on a light rail system that will carry only a few thousand people a day.
A few thousand people on a train means a few thousand fewer cars in the business district. The costs entailed by those cars have to be taken into account to decide whether the proposed scheme is worthwhile. I imagine a single extra parking garage would by itself be, what? One to five percent of the projected costs? Other costs from those few thousand cars will be lumped with other things, like street maintenance, and hard to evaluate.
As I understand it, land acquisition in urban areas is always a substantial part of the costs for these projects. It is very possible that actual operating costs will be substantially lower with rail. On the other hand, a public amenity like rail transit might contribute substantially to increasing land values, and this might be the main reason for some people's support.
Unfortunately for the regular citizen, the information needed for a correct evaluation will probably be hidden away. Whatever information must be public record will be hidden in multiple obscure sources with peculiar jargon and fine print that takes a certain amount of expertise to ferret out and understand.