The long-delayed, $150 million makeover of the busy subway complex beneath City Hall will begin early next year, a top SEPTA official said Wednesday.

The century-old 15th Street station on the Market-Frankford Line and the 87-year-old City Hall station on the Broad Street line will be made roomier, brighter, and cleaner, and be equipped with 13 elevators to make them fully accessible.

And the dark, narrow passageways that connect the stations and the Subway-Surface Lines will be reconfigured to eliminate stairs and bottlenecks.

The dilapidated subway complex is the city's busiest: About 29,000 riders enter the two stations through the turnstiles each weekday, and thousands more use the free interchange that links the subway lines and the trolley tracks beneath City Hall.

Construction will start in March or April 2016 on the 15th Street station, with City Hall station construction to begin later that year, SEPTA deputy general manager Jeffrey D. Knueppel said.

The reconstruction of the 15th Street station is to be completed in 2018, and the City Hall station is to be finished by 2020, said Robert L. Lund Jr., SEPTA assistant general manager for engineering, maintenance, and construction.

The 15th Street station will be kept open during construction, but City Hall station may need to be closed for part of the work, Knueppel said.

"We're looking at what we can do to keep the station in service, but if it really drives the price up or makes the project take much longer, we may have to change that," Knueppel said.

"It's a significant issue. There's not a lot of room there to work with," he said.

The engineering challenge of the City Hall station renovation is compounded by the fact that its massive pillars also support the 548-foot-high City Hall above it.

"It's complicated. You've got the heaviest masonry structure in the world, and the seat of government, up above," said Knueppel. "It's not your routine station project."

The pillars will be reduced in size to create more room for people in the confining City Hall station.

The reconstruction of the subway hub has been on SEPTA's books for many years, but until the state increased transportation funding late in 2013, the money was never available.

"It's something that's long, long overdue," Knueppel said. "The big thing that's so exciting is to finally get elevators. . . . For years, they couldn't figure out how to make it accessible."

It will require three separate elevators to reach the Broad Street Line from street level.

With large crowds expected this summer as the city hosts Pope Francis, and possibly next summer as Philadelphia seeks to host the Democratic National Convention, the transit agency will try to dress up the rundown stations before reconstruction begins.

SEPTA crews will "refresh" the stations this summer by resurfacing the platforms and repainting the walls, Knueppel said.

"We have a lot of guests coming into town. . . . We're trying to keep it as livable as we can," he said.

The overhaul follows on the heels of the $55 million Dilworth Park reconstruction, which opened on City Hall's west apron last fall, with improved access to the subways below.