And this time, it was enough to send the Sounders into the second round of the USL First Division playoffs.

Seattle, having shut out the expansion Railhawks in their previous three meetings, did it again on Sunday night at Qwest Field. Leighton O’Brien’s penalty kick just a few seconds before the end of second-half stoppage time gave the Sounders a 1-0 victory in the second leg of the two-leg, total-goals series in front of 3,640 fans.

Seattle, which took the first leg last Friday night in North Carolina by a 2-0 score, won the series, three goals to none.

“They were not going to let us get into any sort of rhythm whatsoever,” Sounders coach Brian Schmetzer said. “We played hard, we fought hard, the opposition was tough. But at the end of the day, we won the game.”

The Sounders head into the semifinals -- another two-leg, total-goals set -- against the Puerto Rico Islanders, with Game 1 on Friday in Puerto Rico, and Game 2 next Sunday at Starfire Sports Complex in Tukwila. The starting time of both games is yet to be determined.

Tickets for Seattle’s home game go on sale at 10 a.m. Monday.

Puerto Rico, since joining the league in 2004, has not been a friendly place for the Sounders. The Islanders beat Seattle by a 2-0 score in 2005 and 4-1 in 2006. Seattle got its first-ever point in Puerto Rico this past April 29 when the teams played to a 0-0 tie.

“We need to get a result down there, and that’s difficulty,” Schmetzer said. “But I have enough confidence in my guys that they’ve been there and done that kind of thing away from home to give us a fighting chance.”

Seattle blanked Carolina in both regular-season games, 1-0. Counting Sunday’s playoff victory, the Sounders now have won 10 games by 1-0 margin this season.

“We knew they were going to come at us. They’re the kind of team that thrives on work rate,” Sounders defender Danny Jackson said. “We knew we had to stifle them for the first 20 minutes.

With a two-goal cushion coming into the game, the Sounders didn’t need any more goals other than for series insurance. But they played much of the game as if they were the team which had to come from behind. Seattle had four near-misses during the first half and half a dozen more during the second half.

The one that went in was O’Brien’s penalty kick, about 4 œ minutes into the five minutes of stoppage time at the end of the game. The PK was awarded after Kenji Treschuk was taken down in the box by Carolina’s Joey Worthen on a 2-on-1 Sounders breakaway. O’Brien sent his shot into the back right corner.

Carolina goalkeeper Chris McClellan, who ranked second to Seattle’s Chris Eylander in saves during the regular season, almost single-handedly kept the Sounders off the board and kept the Railhawks in the match. McClellan made eight saves on 17 Seattle shots.

The Sounders had four chances during the first half to put the series out of reach, only to be denied each time by McClellan.

The Railhawks, after being kept on their heels by the Sounders through the first 20 minutes of the game, finally started to attack midway through the first half. Eylander saved two well-struck shots: one by Jonny Steele from about 25 yards away in the 29th minute, and one by David Stokes fro the to of the arc midway through the 30th minute.

Sounders 1, Carolina 0 (Sounders win series, 3 goals to none)

First half -- None.

Second half -- 1, Sea: Leighton O’Brien, PK, 90th minute.

Shots -- Car 14, Sea 17. Saves -- Car (Chris McClellan) 8, Sea (Chris Eylander) 7. Corner kicks -- Car 4. Sea 5. Offsides -- Car 3, Sea 2. Fouls -- Car 27, Sea 10. Cautions -- Car: Joey Worthen, 45th minute; Chad Dombrowski, 77th; David Stokes, 84th; Worthen, 90th. Sea: Josh Gardner, 17th minute; O’Brien 27th; Kenji Treschuk 52nd; Hugo Alcaraz-Cueller 77th. Ejections -- Car: Stokes, 85th; Worthen, 90th. Attendance -- 3,640.