There is huge potential for extending at least as far as Healdsburg.
It's a touristy area with small downtown easily accessible without a car just like Santa Rosa Railroad square area
Highway 101 constantly over crowded state would have to spend a lot more on added lanes would face huge opposition and delays.
If SMART and convention and visitors bureaus teamed up and appealed to tourists visiting SF to take ferry to Larkspur and train to Healdsburg and beyond as not the fastest most frequent way, but a scenic less stressful way to get to the Northbay without a car it could attract an affluent non commuter crowd that would spend a lot of money on food and lodging in Petaluma, Santa Rosa and Healdsburg maybe even Ukiah with lunch and dinner trains operated by GGRM or Napa Valley wine train type operation.
People could make a day trip from SF or spend a couple days in the area.
If done and marketed properly It would appeal to wealthy US, European and Asian and other parts of the world visitors.
If you build it they will come.