SEPTA plans to close the 3,175-foot-long bridge that carries Norristown High-Speed Line trains over the Schuylkill, the first in what could be a cascade of cutbacks caused by a lack of money, the agency said Thursday.

The 101-year-old Bridgeport Viaduct will be closed next summer, when warmer temperatures cause tracks to expand and pull free of steel spikes in rotted wooden ties, SEPTA chief engineer Jeff Knueppel said.

The bridge carries about 2,400 passengers a day to and from the Norristown Transportation Center. Buses will be used to carry passengers over an adjacent highway bridge while the rail bridge is closed, SEPTA officials said.

Emergency repairs last month, including gluing spikes in place, will allow the bridge to be kept open until summer, Knueppel said. Train speeds on the bridge have been reduced to 15 m.p.h. from 25.

It would cost about $7 million to replace the 1,708 wooden ties and an additional $23 million to repair and repaint the bridge, he said. That would mean closing the bridge for at least four months.

But SEPTA officials said the bridge may remain closed indefinitely, because they don't have the money to fix it.

And the closing could be followed by the shutdown of other bridges and facilities, as SEPTA's backlog of unfunded construction projects continues to grow.

"This is just the start," SEPTA Board Chairman Pasquale T. "Pat" Deon said Thursday. "There can be a ripple effect out through the rest of the system."

SEPTA relies on state funds for most of its capital budget, the money that goes for vehicles and big construction projects.

Although ridership is at a 23-year high, SEPTA's capital budget is at a 15-year low, at $303 million.

Deon and other SEPTA officials Thursday said a new state transportation-funding plan, for public transit as well as highways, is crucial to maintaining the region's rail, subway, and bus network.

As Gov. Corbett and state lawmakers prepare to tackle transportation funding as part of next year's state budget, SEPTA officials worry that rural lawmakers will push for more funding for highways but not for transit systems.

The Bridgeport Viaduct is the poster child for SEPTA's financial woes.

Last year, U.S. Transportation Secretary Ray LaHood stood in the shadow of the bridge to make a last-minute pitch for an unsuccessful Obama administration effort to get Congress to provide $50 billion for transportation projects.

The bridge was built by the Philadelphia & Western Railway in 1911.

In addition to its rotting wooden ties, its steel is rusting and its concrete piers are cracking.

The viaduct is among scores of crumbling bridges, passenger stations like City Hall, and power substations that make up a nearly $5 billion backlog of unfunded projects.

The state's last effort to provide long-term funding for transportation was Act 44, passed in 2007. But that plan failed to provide the anticipated money because it relied on tolls on I-80, and the Federal Highway Administration refused to grant permission for the state to toll I-80.

Any new plan should be stable, predictable, and able to grow over time, SEPTA chief financial officer Richard Burnfield said.

The state sales tax, which currently sends 4.4 percent of its revenue to transit, is a favorite of transit officials.

But state lawmakers know that if they dedicate more of the sales-tax revenue to transit, they'll have a budget gap elsewhere to fill.