BART called it “the blitz”: For weeks, teams of yellow-vested managers and police officers swarmed the downtown San Francisco stations at dawn, causing would-be fare cheats to turn around and nervously pat their pockets.

By several measures, it was a success. Tickets sales shot up 10%. Police calls dropped 50%. A third more riders added cash to their tickets or Clipper cards, suggesting that many people who gate-hop or piggyback actually carry enough money to pay.

But the crackdown also had an intangible benefit. It gave the agency’s front-line managers a closer glimpse of a problem that riders complain about every day: the thousands of people who sneak in and out of the system without paying.

“I think they were aware of the problem, but this helped reinforce it,” said Board Director Robert Raburn. “And that’s the beauty of this effort. You don’t want ‘Ivory Tower’ managing.”

The effort that began April 8 — and is still going on, though the police and staff have dwindled — was an experiment to see whether extra law enforcement would compel people to pay up. Teams of BART police officers, managers and fare inspectors stood at the gates every weekday morning, starting when the system opened at 5 a.m. and remaining until the rush-hour crowds tapered off at 10 a.m.

The idea: Use the police as a temporary solution while officials scrounge up money to raise railings, put elevators inside station gates and lock emergency gates.

BART may ultimately replace its 600 turnstiles, at a cost of up to $135 million. Board directors are weighing several possible designs, including New York’s “Iron Maiden” — the revolving door bars that come together like interlocking teeth.

“We need a blended approach,” said Director Mark Foley, who favors those gates and other forms of “station hardening,” but saw the blitz as a good law-enforcement model — it demonstrated that BART can deter scofflaws by assigning more officers to its busiest stations.

Data from the first four weeks showed that conspicuous police officers get results. From April 8 to May 3, BART saw add-fare transactions increase by roughly a third compared with the previous four weeks — from 953 to 1,233. Vending machine sales ticked up from 3,235 fares before the blitz to 3,575 during the month of extra enforcement. Proof-of-payment citations also jumped from 270 to 306. At the same time, police calls went down almost 50%, from 22 a week to 12.

“Police calls going down was the most important piece of data,” said board Director Debora Allen. She’s expressed frustration with BART’s inability to solve fare evasion and what she sees as an effort to downplay the numbers.

To Allen, the downtown San Francisco operation showed the magnitude of a problem the agency has known about for years but failed to quantify. BART’s management says cheats siphon $15 million to $25 million a year in revenue, but some directors believe the losses are higher. A former top official estimated that 4% to 5% of riders don’t pay, so if the average ticket price is $5.50, the agency bleeds up to $35 million a year.

None of those raw estimates takes into account the larger cost of the fare-cheat epidemic: It taints BART’s public image and erodes public trust. Many people associate cheats with the increased visibility of homelessness, drug use and crime in the transit system. Whether that perception is justified or not, it has caused riders to peel away: BART’s ridership has fallen 3.4% since 2016.

The downtown blitz was the brainchild of Officer Keith Garcia, who heads BART’s Police Officers Association. He persuaded transit officials to put extra police in downtown San Francisco after they approached him with a similar proposal for the end-of-the-line stations, which would have been less effective, Garcia said.

He called the effort an “eye-opening experience,” particularly for managers who normally work out of BART’s headquarters near Lake Merritt in Oakland. Some of them were genuinely surprised to see how many people push through or vault over turnstiles, sneak down the elevator or blow through the side gates.

“They should have known how bad this problem was long ago,” Allen said. “I’m glad we woke them.”

At points, even Garcia was startled by the volume of fare evaders. One morning he stood for four hours at a stairwell near an elevator, one of many portals that people use to avoid the turnstiles. He counted 103 people who tried to come down the stairs that morning and turned around when they saw a police officer.

Spokeswoman Alicia Trost said BART will keep the crackdown going indefinitely, even though the agency has yet to calculate the cost of the experiment. It required 16 police officers each day, some working regular shifts and others on overtime. It also diverted managers away from their regular jobs so they could stand at the gates in bright yellow vests, which Trost said added no cost for taxpayers.

“We want to be nimble and shift staffing levels and try new things beyond station hardening and changes to our fare gates,” Trost said, referring to two separate campaigns to make the transit system less porous. BART devoted part of its capital budget to raise railings, lock swing gates and put elevators inside the paid areas of stations, closing off some of the portals people use to sneak inside without paying. It also began modifying the existing fare gates, adding metal plates that pop up to discourage jumping, or increasing air pressure to make them harder to pry open.

Much of BART was designed in the 1960s and 1970s, by politicians and transit leaders who wanted a roomy, suburban feel. Societal changes forced the agency to rethink that aesthetic: now the board is adding new security infrastructure to gird passengers against homelessness and injection drug use. It’s a painful acknowledgment that BART lacks the resources to help people in obvious despair, so it’s instead creating barriers to keep them out.

As passenger complaints mount, BART’s board has walked an awkward political tightrope. Directors want to show the agency is taking fare evasion seriously, while also appearing compassionate to people who seek shelter in the transit system.

“We’ll spend almost $20 million this year creating a better customer experience,” said Director Lateefah Simon, referring to BART’s efforts to modify fare gates and “harden” stations. “But what we really have to do is organize the local elected leaders whose responsibility it is to house and feed people.”

“When the elected leaders fail, all of those folks wind up on the subway,” she added. “A train system will never have the capacity to deal with a regional human rights crisis.”

The budget that goes before BART’s board next week features a patchwork of strategies to quell fare evasion: station ambassadors, higher railings, elevator attendants, outreach teams and 19 new police officers. Eventually, most officials hope BART will build enough protective infrastructure to force people to pay.

But that wouldn’t completely substitute for a visible police presence, Raburn said. Some passengers will always want the “human touch” of a person guarding a gate.

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