Rail service from the North Bay to San Rafael via SMART trains will begin next Friday, transit authorities said this week, giving commuters another option to help avoid traffic gridlock.

The Sonoma Marin Area Rail Transit trains will begin full service at 12:49 pm on Aug. 25 and fares will be 50 percent off until Sept. 4. Riders must use their Clipper cards or smartphone to pay fares, which range from between $3.50 to $11.50 for a one-way trip.

The rail line that begins service Aug. 25 will run…