Sound Transit is adding a mid-day Sounder train to its popular south line service beginning Monday, Sept. 12. The new train will provide flex-schedule workers and other riders an additional travel option for bypassing highway gridlock that now snarls traffic throughout the day.

The mid-day train will depart Lakewood at 10:18 a.m. and stop at all south line stations before arriving in Seattle at 11:31 a.m. The return train in the afternoon will depart King Street Station at 2:32 p.m. and arrive in Lakewood at 3:45 p.m.

It will include a stop at the Tukwila station, just west of the border with Renton.

This mid-day Sounder train will bring to 11 the number of daily trains serving the popular south line Monday through Friday. Eight of these trains will operate as northbound trips to Seattle starting from Lakewood at 4:41 a.m., with return trips leaving King Street Station beginning at 3:12 p.m. Two reverse-direction trains leave Tacoma for Seattle at 6:15 and 6:50 a.m. each weekday morning, and return to Tacoma at 4:30 and 5:00 p.m.

The new mid-day train will start as a two-car train, and increase to seven cars when nine new Sounder coaches arrive early next year.

The additional service comes as ridership on the Sounder continues growing in double digits. This year, second quarter ridership on the south line jumped 12 percent to the same period last year.

Voters approved four additional Sounder south line trains in the 2008 Sound Transit 2 ballot measure. The first roundtrip began serving Lakewood in 2013. Two additional peak-service trains will begin service in 2017.

A complete schedule of south line Sounder service starting Sept. 12 is available at soundtransit.org/schedules.