Minneapolis and St. Paul are considering a number of streetcar lines. Three have risen to the forefront. The Central/Nicollet Line in Minneapolis, the Midtown Greenway in Minneapolis, and the Seventh Street Streetcar in St. Paul. In part this is viewed as an economic development tool. In part this nostalgia for an earlier simpler time, when we were all children, growing up in Pittsburgh.
As with anything, resources are not infinite (I wish they were, but if wishes were horses we would all be riding around on horses). Thus there are trade-offs.
The general trade-off is that streetcars are more expensive than buses, particularly in initial capital expenditures. Thus with a finite amount of resources you can do fewer streetcars or more buses. Those rail-based rides are slightly nicer (particularly smoother) (since with streetcars you are building some of your own infrastructure, while with buses you are literally free-riding on infrastructure built by someone else, in various stages of disrepair).