PHILADELPHIA - A tentative contract agreement has been reached that could end the public transit strike that has idled Philadelphia's subways, buses and trolleys for four days, Gov. Ed Rendell said late Friday.

"I'm very optimistic that the trains and buses will be in operation tomorrow evening," the Pennsylvania governor said late Friday night at a news conference.

Rendell, who has been brokering the talks, said negotiators for Transport Workers Union Local 234 and the Southeastern Pennsylvania Transportation Authority tentatively agreed on a proposed contract and both sides are set to take a vote at 2 p.m. Saturday.

"We hope to be announcing the resumption of service by Saturday evening," SEPTA spokesman Richard Maloney said late Friday.

The union represents about 5,000 bus drivers, subway and trolley operators and mechanics. They walked out early Tuesday in a dispute largely centered on pension benefits.

The union had threatened to strike while the World Series was in town last weekend, but negotiators continued bargaining after Rendell threatened "significant consequences" if that happened. The union went on strike hours after the series between the Phillies and Yankees shifted back to New York.

SEPTA's regional railroad is still running because those workers are represented by a different union, but that system has experienced problems of its own this week.

On Wednesday, a railcar caught fire as it headed downtown, causing delays and confusion but no serious injuries. On Thursday, a packed commuter train struck and killed a rail worker during the morning rush, stranding hundreds of riders as lines had to be shut down for hours.

Neither accident was related to increased volume due to the strike, SEPTA said.