First some good news: Bike thefts on BART have steadily been decreasing over the past two years.

The bad news? Cycling advocates and BART officials say bikes secured with sturdy U-locks, once thought the surest way to thwart would-be thieves, are increasingly becoming a target.

“The newer pattern … is disturbing,” BART's bike program manager, Steve Beroldo, said at an August meeting of the BART Bicycle Task Force . “U-locks are not impregnable; you can get through them, and unfortunately, people have learned how to do that.”

It's especially troubling, Beroldo said, because the transit agency has been telling people to buy U-locks, which are thicker, more durable and harder to foil — at least in the past. Between 2014 and 2015, BART partnered with bicycling advocacy organization Bike East Bay , paying the organization between $50,000 to $60,000 to set up information tables at stations, distribute theft-prevention fliers and hand out coupons for the U-locks valued at $40 or more, Beroldo said. The transit agency also installed signage informing people how to properly lock their bikes with U-locks, in an effort to encourage cyclists to switch from cable locks to cut down on bike thefts.

“We've done a really good job in getting the word out, telling people, 'Don't lock your bike with anything but a U-lock,' ” BART Director Robert Raburn said.

And those efforts paid off. Between 2014 and 2015, bike thefts fell a whopping 28 percent across the district, even while the number of people riding their bikes to BART soared. That trend appears to be continuing in 2016 — data provided by BART police shows bike thefts were down 22 percent for the first seven months of the year, compared to the same time period last year. Robert Prinz, an outreach coordinator for Bike East Bay, credited the public information campaign, along with serious investment from BART to install bike lockers at stations and team up with cities to open valet bike parking, for those declines.

Except now it appears that U-locks are becoming more of a target. Prior analysis by BART police indicated that roughly 80 percent of bike thefts on BART properties involved bikes secured with cable locks. A recent analysis by this newspaper of bikes reported stolen in the first three months of 2016, revealed that, where the type of lock was mentioned, close to 45 percent involved a cable lock, while nearly 38 percent involved U-locks. BART was unable to provide incident-level data for the months of April through July, because the agency changed its data management system in April, said Jason DeVera, BART police's administrative traffic supervisor.

Of the stolen bikes secured with a U-lock, slightly fewer than one-third were secured with either a U-lock and a cable lock or two U-locks. In at least one case, the cyclist secured the bike with two Kryptonite U -locks — among the most expensive around — and a Kryptonite-brand chain, only to find the bike missing.

The Dublin/Pleasanton, Fremont, and North Berkeley stations had the most bike thefts for the first seven months of 2016. Of those stations, only the Dublin/Pleasanton station was in the top three stations for bike thefts in 2015, when it joined the Pleasant Hill and Concord stations. North Berkeley and Fremont made the top three in 2014, along with MacArthur station.

Prinz said the trend, though troubling, is not entirely surprising: “As people get more savvy about how they lock up their bikes, others get savvy about how to break those locks.”

BART is planning to experiment with a new type of bike rack, called Bikeep , which uses an industrial-grade steel bar to secure bikes to the rack. Two sets of those racks will be installed early next year at the 16th Street and Pleasant Hill stations , Beroldo said, and users will be able to lock and unlock the bike with their Clipper card.

But Beroldo said Bikeep “may just be the tip of the iceberg” in terms of new technology the agency is willing to deploy to keep bikes safe. And Raburn said the agency is actively seeking ways to expand the number of bike racks and bike lockers at stations wherever there is room and available funding. He's also hoping that with the expansion of Bay Area Bike Share into the East Bay, cyclists might consider leaving their bikes at home or not bringing them on trains.

BART plans to expand the number of bike stations, including valet and self-serve bike facilities, Beroldo said, beginning with MacArthur, which will receive a new self-serve station early next year, and Pleasant Hill, which will receive a hybrid valet and self-serve station, by the spring of next year.