SEPTA is offering a glimpse inside the Center City trolley tunnel as crews work to fix problems that have shut down service for weeks, forcing tens of thousands of riders onto detours.

The tunnel has been closed since mid-November after equipment failures damaged the overhead power infrastructure. SEPTA officials now say the tunnel will remain closed through the end of the year, with hopes of restoring service sometime in January.

On Wednesday, SEPTA allowed cameras inside the underground system, giving reporters a ride-along on a test trolley moving through the tunnel.

"We're going to demonstrate how they check carbons, how they replace carbons," Kate O'Connor, SEPTA's assistant general manager of engineering, maintenance and construction, said, "and we're also going to walk through some of the things that have been done to the overhead catenary system to date."

SEPTA says the issue began in October after the agency installed a new replacement part intended to make maintenance easier and less expensive. Crews replaced 3-inch sliders, which connect trolleys to overhead power lines, with larger 4-inch versions.

Instead of improving performance, the larger sliders caused carbon inserts to wear out quickly, damaging the tunnel's overhead wires, SEPTA officials said.

"Unfortunately, the underside of the wire is very rough from that interaction with the 4-inch slider," O'Connor said, "and we're still seeing issues with the carbon wear because of that."

Crews eventually switched back to the original 3-inch sliders, but on Nov. 13, they discovered additional damage. Service was halted again later that day.

"The trolley that stopped at 19th Street had completely lost its carbon, was worn all the way through," Jason Tarlecki, SEPTA's acting chief deputy engineer for power, said. "And that's when we had to stop the service and reassess the damage."

Since then, SEPTA engineers have been testing how frequently carbon inserts need to be replaced and smoothing out the damaged overhead wire using a brass burnishing tool. During Wednesday's test run through the tunnel, the trolley ride appeared largely smooth, though crews continue to monitor performance closely.

The shutdown has affected an estimated 60,000 to 80,000 riders each day. Trolley routes have been diverted to 40th Street. SEPTA has added shuttle buses along Market Street to help accommodate displaced riders.

"We do apologize for the inconvenience that's caused by having to use the diversion route," O'Connor said. "We don't want to rush to reopen because we're not trying to get back to service in the tunnel before we're sure that it's safe to do so."

SEPTA officials say additional testing will continue in the coming weeks as crews work toward reopening the tunnel early next year.