The measure was getting 68.6 percent approval of the combined number of voters in both counties, according to unofficial results early this morning. It needs two-thirds approval to pass.
The measure failed by 1.4 percent in 2006. It also passed handily in Sonoma County but failed in Marin County the last time it was on the ballot.
Sonoma-Marin Area Rail Transit, or SMART proposed the measure. The train will run 70 miles along the existing Northwestern Pacific Railroad line. A bike and pedestrian path will run parallel to the route.
Proponents say the train will take 1.5 million vehicles a year off the U.S. Highway 101 corridor, reduce greenhouse gas emissions by 124,000 pounds every weekday and reduce personal carbon emissions by 70 percent.
Opponents have called the proposal a "train to nowhere," questioning whether it will attract enough daily riders to make it worth the expense.
The estimated capital cost of the train and 14 stations is $450 million and the adjacent bike and pedestrian path is $90 million in 2008 figures.
The tax will raise almost $900 million over 20 years.