Splashy extensions in recent years have taken BART down the Peninsula, deeper into the East Bay and tantalizingly close to Silicon Valley. Meanwhile, its core is falling apart.

Some transit observers — and even some of the people who run BART — say the attention the agency lavished on shiny new stations and rails into fast-growing suburbia distracted it from the health of the system’s heart: its original stations, its train control system and its rail cars.

“All of the extensions we built from the core system without being able to maintain the core system have been a mistake,” said Joel Ramos, regional planning director for TransForm, a transit advocacy group that fought the Oakland Airport Connector, BART’s extension to Oakland International Airport. “If the core system is not maintained, the extensions are good for nothing.”

The crisis that hit BART last week — with dozens of train cars disabled by mysterious power surges, service shut down between the North Concord/Martinez and Pittsburg/Bay Point stations, and angry riders being squeezed into shortened trains — has drawn attention to whether the agency is spending enough time and money on the unglamorous task of keeping infrastructure in working order.

Where the money was

When it came to ignoring its nuts and bolts, BART had plenty of help.

For BART and other transit agencies, the big money available in the 1990s and early 2000s was for expansion, said Randy Rentschler, spokesman for the Metropolitan Transportation Commission, the Bay Area’s transportation planning and financing agency. Federal, state and even local governments were interested in funding rail extensions, not maintaining or rebuilding existing systems, he said.

“Money was designed for extensions exclusively,” Rentschler said. “In almost every case, it focused on expansion. In almost no case did it focus on maintaining what we had in those days.”

Around the Bay Area, the idea of extensions was also popular. After cities were built up in the far eastern reaches of Contra Costa and Alameda counties, BART got a line that runs out to Pleasanton and laid plans for an extension to Antioch that is due to open in 2018.

On to Millbrae

San Francisco wanted the system to serve San Francisco International Airport, and San Mateo County, even though it wasn’t part of the BART district, wanted some stations in its cities on the way to SFO. That’s why trains now run to the airport and to Millbrae instead of the line ending at Daly City.

The money San Mateo County paid to help make that happen brought BART not just to the Livermore Valley, but also to Bay Point and the soon-to-open Warm Springs Station in southern Fremont.

Ridership boomed: The 440,000 patrons who take BART on an average weekday represent a 25 percent increase in just five years. But BART’s expansions didn’t come with any additional money to buy more rail cars or modernize increasingly busy stations like those in downtown San Francisco.

In the budget year that starts July 1, BART plans to spend $120 million from its $1 billion operating budget on capital projects, including new rail cars, a train control system and a maintenance yard expansion.

The feeling on BART’s elected Board of Directors was that the maintenance issues were manageable, Rentschler said. Directors, who set the system’s priorities and oversee the transit professionals who make the trains run, jockeyed to get extensions in their districts built first.

“That was just the business model at the time,” Rentschler said. “There was a culture of trying to complete the BART system, a sense that this was the right thing to do.”

Stretched thin

Board of Directors member Tom Radulovich of San Francisco was on the board through some of the expansion years and was an early, and often lonely, voice for pumping money into the core system.

BART’s mistake with the extensions, he said, was that it stretched its resources thin. There were no new rail cars, no new maintenance shops or train yards, no expansion of downtown San Francisco stations’ capacity.

“It was not wrong to expand,” he said. “It would be great to serve all corners of the Bay Area, but we can’t expand at the expense of the existing system. And that’s what BART did.”

BART learned some lessons from those mistakes, Radulovich said. As part of a deal to extend its rails south from Fremont to San Jose, BART negotiated a buy-in deal with Santa Clara Valley Transportation Authority that called for the VTA to pay for and build the extension, cover its operating costs, buy new rail cars, help expand the Hayward maintenance facility, and pay for other effects on the core system.

“We’ll have to do that with future extensions,” Radulovich said, “even within the BART district.”

BART is also trying something new in far eastern Contra Costa County, building an extension to Antioch that uses cheaper diesel-powered rail cars. It will cost in the neighborhood of $500 million, instead of the $1.2 billion it would have taken to extend the existing system east of Bay Point.

Oakland airport link

Among the most controversial extensions of recent years was one of the shortest: the 3.2-mile Oakland Airport Connector. Transit advocates, including TransForm, battled the airport link, saying the $484 million it cost would have been better spent on new BART cars and maintaining the aging parts of the system.

Joel Keller, like Radulovich a 20-year veteran of the BART board, pushed for the earlier extensions and supported the Oakland airport link. Now, he said, he realizes the need to make reinvesting in the existing system a priority. He said that was clear to him even before last week’s breakdown crisis, which affected thousands of his constituents in eastern Contra Costa.

“I have evolved to the point that I think we need to invest in the core system until it is reliable and dependable again,” Keller said. “Then we can think again of other priorities.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan