It was supposed to be BART’s shining evolutionary moment. Instead, it’s been a year and half of stumbles and delayed gratification: The system’s new railcars are grinding through maintenance issues, slowing the rollout of a fleet that captivated riders and transit officials.

One example is the “D” cars, which have a cab where the operator sits. The agency expects them to run 6,000 hours before hitting any kind of equipment failure that causes a delay of five minutes or more. They’re hovering at 1,000 hours.

The “E” cars that line the middle of the train are performing better, at 9,745 hours between each major disruption. But they’re still short of the 12,000 benchmark.

“It’s a pretty high hurdle,” said Chief Mechanical Officer Dave Hardt. He hopes to see a breakthrough around the time BART receives Car 100, though no one knows when that will be. The eighty-fourth car arrived last week, more than a year after BART premiered its first Fleet of the Future train on the Richmond line. Seventy-five cars have been certified by the California Public Utilities Commission, the equivalent of 7½ trains.

The transit agency is fighting against time as it retools the silvery new rail vehicles that superficially resemble BART’s old trains — same aluminum surface, same four-wheeled “truck” with a motor and brakes — though their innards are completely different. Last year BART aimed to have 775 new cars in service by 2022, but in a recent interview, Hardt pushed the date to spring of 2023.

“There’s a bit of a horse race going on, with BART opening new extensions, including the one to Warm Springs and the one to Berryessa” in San Jose, said Tom Radulovich, a former BART board director who now heads the transportation policy group Livable City. “So it needs to accommodate a growing ridership on a larger system with more miles of track.”

Fleet of the Future trains started to appear on some East Bay lines about a year ago, but they were as rare as unicorns or four-leaf clovers: every sighting of a new train, with its bike racks, chartreuse seats and flashing destination signs, seemed to warrant a photo on Instagram.

By October, they were crossing the bay — a huge development that signaled BART’s confidence in the new cars. No longer did the agency fear they would break down in the middle of the Transbay Tube and choke the regional rail system.

Since then, progress has slowed, as BART prolongs the testing period so that its manufacturer, Bombardier, can meet stringent contract specifications. D cars are supposed to operate for 500 hours before a failure of any type — they currently average 56 hours. E cars run 149 hours between incidents, well below the 700-hour goal.

BART gets to withhold the last 5 percent payment for the $2.5 billion fleet until both sets of cars hit their maintenance targets, which is why officials are so obsessive about documenting everything, Hardt said. If Bombardier never meets its performance measures, BART will save about $100,000 per car — and struggle through the problems on its own.

Procuring new train cars is by nature complex and time-consuming on a system as idiosyncratic as BART. Its engineers are stuck with a lot of decisions that were made in the 1960s, such as the 1,000-volt operating system and the height and width of the Transbay Tube. Because BART doesn’t use a standard-gauge railway — a term denoting the international norm of 1,435 millimeters between rails — all of its equipment has to be custom-built and tested on BART tracks, not in a facility.

Officials say the Bombardier cars are steadily improving, even if they spend a lot of time in the shop. From July to April 15, BART tallied 3,363 work orders for the trains. For most of that time, the transit system had fewer than 50 new cars.

The 662 aging legacy cars had similar repair needs, producing 48,544 work orders over the same period. Yet they go about 4,800 hours between service delays — a rate five times more reliable than D cabs, which are supposed to carry BART though an era of intense population and job growth.

So far, that discrepancy doesn’t worry Hardt.

“Don’t forget it took years of engineering and maintenance to get to that number,” he said, noting that the “legacy” trains were half as good when he started working at BART a decade ago. At that time they ran 2,400 hours between breakdowns. Before that, the numbers were abysmal: It took less than 1,000 hours for a train to fail.

It’s common for new railcars to scramble through a period of mishaps. In San Francisco, Muni’s new subway fleet has damaged couplers and issues with its door sensors, which have both hobbled the transition. The San Francisco agency is running one-car trains and locking the rear doors for the time being.

Most of BART’s challenges stem from 180 software packages and 30 microprocessors that make up each Bombardier vehicle. Each is a complex symphony of systems that communicate over an Ethernet backbone. A bug in one line of code could shut the whole thing down, requiring a rigorous safety review process that may take months before engineers can reinstall the software.

Some problems are big and intricate. Occasionally, engineers find faulty code in the vehicle automatic train control system, which operates the car, Hardt said.

Still, he and other officials are optimistic about the new internet of BART — a significant leap from the 1960s design in which all trains looked like carpeted airplanes. Ultimately, staff sitting in a control center will be able to track the health of each car, from the brakes to the air conditioner. They won’t have to rely on a passenger reporting a piece of broken equipment and hope that passenger gets the car number right.

“I don’t know everything these new trains can do — they probably have artificial intelligence or something — but I know they’re better than these buckets of steel we’re sitting in,” said Board President Bevan Dufty.

BART has coped with technical issues by drawing out its acquisition and testing period. When its Board of Directors awarded the contract to Bombardier in 2012, it expected to put new cars on the tracks by 2017 and have 410 futuristic trains in service by 2020. In reality, the trains premiered a year later than anticipated, and with seven months left in the year, BART is nowhere near the 410-car benchmark.

Stretching the timeline has allowed BART and its contractor to work through failures. But it’s been a pain for passengers who have to endure old, clunky cars, some of which date to the 1970s.

That’s a trade-off the agency had to make, said Randy Rentschler, legislative director of the Metropolitan Transportation Commission, which manages transportation planning and projects throughout the nine-county Bay Area.

“BART made the right decision to extend testing of these cars and get them manufactured in the best possible shape,” he said. “These new cars are crazy, complicated electronic marvels. They’re not like old Muni streetcars” that just have wheels and go.

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