PITTSBURGH — Linebacker Joey Porter says Jerome Bettis is pro football's equivalent of baseball's closer: The Pittsburgh Steelers never seem to use him when the season starts, but he's always there at the finish.

Bettis is the NFL's No. 5 career rusher with 13,581 yards, and no back close to his 255 pounds has gained nearly as many yards. That hasn't prevented the Steelers from starting the last three seasons with, in order, Amos Zereoue, Duce Staley and Willie Parker as their starting back.

It's funny that when the temperature drops, the snow flies and the games get important, the starter gets pushed aside and one man ends up with the ball: Bettis, who has 30 of his 61 career 100-yard games during the second half of a season. That's unusual given how fatigue and injuries often catch up to star runners past midseason.

Bettis, barely used this season until Sunday, proved he had at least one more big game in him when he ran for 101 yards and two touchdowns on 17 carries to keep the Steelers in playoff contention with a 21-9 victory that ended the Chicago Bears' eight-game winning streak.

Right tackle Max Starks couldn't help but laugh afterward as he related how a Bears defense that has allowed the NFL's fewest points and yards couldn't believe how a soon-to-be 34-year-old who previously had 186 yards this season ran over them.

"Those guys weren't anticipating Jerome being so strong," Starks said. "A couple of guys were like, 'Hey, he's too old to be doing that out on the field.'"

Bettis had a 39-yard run and also carried star linebacker Brian Urlacher on the final two yards of a 5-yard TD run that made it 21-3. That didn't surprise Bettis, who said, "I've made my living carrying people — that's why they call me The Bus."

Keeping with the mass-transportation analogy, Bettis was asked if he ran out of gas on his long carry. That brought a laugh and this admission: "I never had much gas. My job is to get all the yards I can and live to fight for another day."

Certainly, Bettis took advantage of a muddy and snow-covered grass field that slowed the Bears' faster and younger defenders. By comparison, he was held to 9 yards on six carries two weeks earlier on the Indianapolis Colts' fast indoor track, and he has had six games of 28 yards or fewer this season.

Still, underestimating Bettis' durability and productivity has proven costly to opponents before.

Last year, Bettis appeared to settle into the role of goal-line back, and he had one of the must curious statistical lines in NFL history in the season opener against Oakland: five carries for 1 yard and THREE touchdowns.

But when Staley got hurt at midseason, Bettis stepped in and ran for 149 yards against the Eagles. Bettis had six 100-yard games in as many starts last season, with a seventh against the Jets in the playoffs. He may have missed an eighth career 1,000-yard season only because he was held out of the final regular season game with an injury.

Regardless, Bettis needed weeks to talk himself into returning for this season, doing so mostly because the Super Bowl is in his hometown of Detroit and he felt the Steelers had a good chance of going.

That's why another 100-yard game hasn't changed his mind about 2006, which means this is almost certainly his last season.

"This is one game, and I was happy to come in and do what I did for my teammates, but it doesn't change anything," he said. "This is Willie Parker's opportunity, and I'm backing him up. I'll keep doing the things I'm doing, and hopefully when they call my number again, I'll be ready."

Is there any doubt he will be? Bettis will finish below 1,000 yards for the fourth consecutive season, yet has far more carries and far more yards than some of the other power backs of their day — 4,984 yards more than Jim Taylor, 4,174 yards more than Earl Campbell, 2,229 yards more than John Riggins.

"Whenever you go into a situation where it could be your last play, there's a sense of desperation," said Bettis, who is aware any carry could be his last. "I definitely feel it, and I try to encourage my teammates and lead by example. It doesn't matter how we do it, all that matters is we get it done."