Violent crime on BART more than doubled since 2014, driven in part by a fare-evasion epidemic that is three times worse than the agency’s official estimates, according to a new grand jury report.

The report released Monday by the Alameda County Civil Grand Jury found the number of robberies on the transit system increased by 128%, from 153 in 2014 to 349 last year. Aggravated assaults soared by 83%, from 71 to 130 over the same time period.

Robberies and aggravated assaults combined jumped nearly 16% from 2017 to 2018 alone, according to the report, with robberies spiking 20% and aggravated assaults rising 7%.

The numbers give credence to rider complaints about the lack of security and safety in the system, which have caused BART’s approval ratings to tank. Over the past three years, ridership has dropped by 8%, from a 2016 peak of 129 million to 118 million projected this year. Officials expect the numbers to plummet to 116 million next year as more people shift to Uber and Lyft.

“BART is at the center of the Bay Area’s transportation upheaval,” the grand jury report concluded. It noted that the sprawling transit system has become a laboratory of societal failures, sheltering homeless people and transients and providing easy portals for scofflaws.

Many transit officials link the rising crime rate to rampant fare-beating on the rail lines, which according to the report is significantly higher than BART previously stated. A senior manager at the transit agency told the grand jury that about 15% of riders do not pay their fares — or 17.7 million passengers out of 118 million. BART officials estimate that about 5% of riders evade fares.

Assuming the average fare is $4.50, the agency could lose nearly $80 million annually to cheaters. Previously, top BART officials pegged their losses at $15 to $25 million.

BART General Manager Grace Crunican wrote in a letter to the grand jury Monday, obtained by The Chronicle on Tuesday, that 15% was not accurate.

“BART has never estimated its fare evasion rate as l5%,” Crunican wrote. “Although fare evasion is challenging for transit operators to measure, prior BART analysis has estimated our fare evasion at around $15 million to $25 million annually, which translates to about a 5% rate.”

“A lot of these crimes are people getting their phones snatched by juveniles who sneak into the system, and then ride the train to San Francisco or Oakland, where they can disappear really quickly,” said Officer Keith Garcia, president of BART’s police union. “Usually there’s a team of two or three or four, so when people fight to keep their property, the people stealing have backup.”

His solution: assign officers to patrol every station and put 6-foot railings around the paid areas.

“The bottom line is we need to secure the system,” Garcia said.

The new fare-evasion figures startled board President Bevan Dufty, who said he shared the grand jury’s concerns.

“I want to get the forensics on where this is coming from,” Dufty said. “When you look from $15 million to $80 million, those are different magnitudes.”

Director Debora Allen was dismayed but not surprised by the numbers. For two years, she has pressed the agency to explain its methodology for determining how many don’t pay.

“This is a crisis,” she said.

Pressure began mounting on BART after an 18-year-old woman was fatally stabbed on the MacArthur Station platform last year. She was among three homicide victims during a particularly violent year for the rail system — no one was killed in BART in 2017. There have been no homicides this year on BART property.

Stung by public criticism and news headlines, BART started an emergency patrol plan last summer, requiring police to work mandatory overtime for three weeks after the slaying of Nia Wilson. The agency has also begun a five-year plan to hire 94 officers, or 19 a year. Last year, the board approved a new union contract with a 16% pay raise, which helped attract new candidates to the short-staffed department.

Despite the recent mayhem and growing rider apprehension, BART also had some good news over the four-year time period: Property crime dipped. Auto burglaries decreased by 32% in BART station parking lots — from 522 in 2014 to 354 last year. Theft, which represents the bulk of nonviolent property crimes, essentially flatlined. In 2014, BART passengers reported 2,597 thefts of phones, wallets, laptops, bicycles and other valuables. Last year, that number hovered at 2,590.

Still, the statistics illuminate the problems of a transit agency that is struggling to restore trust from commuters and taxpayers.

“A growing and far-flung urban population in need of transport to work, home, shopping and socializing has many modes from which to choose,” the grand jury report said. “Rising dissatisfaction with crime on BART, fare evasion, and the perception of dirty train cars and stations threatens to marginalize the agency amid the other choices available to riders.”

It recommended the agency accelerate the hiring of patrol officers to make riders feel safer and continue its crackdown on cheats.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan