SEPTA officials said union workers made no contributions to their pension fund until the late 1990s. Since then, contributions have been negotiated to pay for increases in pension benefits. Union workers now contribute 3.5 percent of their pre-overtime pay to the pension fund.
Sunday's vote authorized a strike but did not set a timetable for any potential work stoppage. Brown said that workers would not strike this week and that the union would reevaluate the situation at the end of the week after more negotiations.
"If we are still as far apart as we are now, we will have no choice but to strike, but we will see if we can get a little closer," Brown said.
SEPTA spokeswoman Jerri Williams said the union vote was no surprise. She said SEPTA management remained committed to avoiding any service disruption.
"We've had several meetings last week and the week before, and we hope to be able to continue discussing the issues and to be able to come to an agreement," she said.
Some of the union members wore T-shirts that read: "We can and will strike!"
"We have to stop giving back. It's just take, take, take," said Joe Pizzo, a trolley operator whose route runs through Southwest Philadelphia.
Negotiations, which have been stuck for more than six months, are set to resume Tuesday.
TWU Local 234 is the largest of SEPTA's 17 unions, representing about 5,000 drivers, operators, mechanics, cashiers, and other workers.
Its members have been working without a contract since the previous five-year pacts expired in March and April.
Brown has scheduled a news conference for Monday to speak about negotiations and the impact of the strike vote.
Typically, any agreement TWU Local 234 reaches for its city workers sets the pattern for SEPTA's 16 other labor contracts.
The TWU last went on strike in 2009, seven months after its contract expired.
That surprise predawn walkout left thousands of commuters in the lurch and drew fire from Mayor Nutter and then-Gov. Ed Rendell. The strike lasted six days.
The base salary for new SEPTA bus drivers is $33,887, and drivers with four or more years of experience are paid $55,620 a year.
Including overtime pay, the typical TWU member makes $64,847 a year, SEPTA spokeswoman Williams said.
The threat of a total shutdown of the SEPTA system was averted when Regional Rail workers came to terms this month with SEPTA after five years of negotiations.
If bus, subway, and trolley workers strike, commuter rail trains will continue to operate.
Inquirer staff writer Tricia Nadolny contributed to this article.