The strike will affect hospital, airport and retail workers, although the full effect would not be felt until Monday's rush hour.
The last regional rail strike, in 1983, lasted more than three months.
"I hope it doesn't go that far. I don't anticipate that it would, but I don't know how long it will take us to try to find a common ground — if there is any," said Stephen Bruno, vice president of the Brotherhood of Locomotive Engineers and Trainmen.
SEPTA said that its offer to keep a previously announced wage increase in effect during an extended two-week cooling off period was rejected by the unions. Bruno noted that the union has been working without a contract for four years and an extension "without any movement toward closure is really pointless."
Bruno said striking workers are seeking raises of at least 14.5 percent over five years — or about 3 percentage points more than SEPTA has offered.
The labor conflict came to a head this week after SEPTA announced it would impose a deal beginning Sunday. Terms include raising electrical workers' pay immediately by an average of about $3 per hour; the top wage rate for locomotive engineers would rise by $2.64 per hour.
SEPTA, meanwhile, is planning to have extra subway cars and trolleys in service.
The strike adds to commuting headaches in the region, where major construction projects are making it more difficult than usual to get around.
The lines carrying PATCO commuter trains between Philadelphia and southern New Jersey are being replaced over the Ben Franklin Bridge, affecting not only the train schedule but also car traffic on the busy bridge.
Emergency work on a bridge on Interstate 495 in Delaware is expected to keep a stretch of that thoroughfare closed at least through the summer, and is forcing additional traffic onto I-95. Additionally, work is scheduled to begin next week on I-95 just north of downtown Philadelphia.
Associated Press writer Marc Levy in Harrisburg and Maryclaire Dale in Philadelphia contributed to this report.