Wiesbaden is one of the larger cities in Germany....without a subway or S-Bahn. The city functions with a bus service.....which over decades has functioned well enough. No one has ever voiced an urgent concern to go beyond that point.
There is a discussion by the political folks in town over an idea for a light-rail or S-Bahn operation for the city. Cost? Well....just for roughly eleven kilometers....or 7 miles....it's figured around 161 million Euro. The city might get between sixty and seventy percent of the money from the state or federal folks.
The odds of this paying itself off? An article from the local newspaper....the Kurier....indicates it's a fairly losing scenario for a while.
The political folks are on the positive side of this....thinking long-term.....it's a positive. Having been around the city for a while.....I've become like the locals....getting used to bus operations. At best... there's likely to be two or three light-rail lines....running three miles max. The vast majority of local folks would still ride buses every single day. Some not ever....in their entire life....ride the light-rail.
My humble bet....it'll happen no matter how the locals feel. And everyone will wonder why it's never paying for itself.