Metro is about a month away from finishing tests on its new generation of subway cars, known as the 7000 series, and expects that an eight-car train made up of “7Ks” will start carrying paying passengers early in January, officials said.

And the end of testing will mean the start of mass production of 7Ks, the transit agency said, with 56 of the advanced-technology cars set to be delivered to Metro before next summer. Hundreds more are due to follow in subsequent months.

Metro hopes to have 748 of the 7Ks operating in its rail network by 2018, although funding for all of them has yet to be approved by the Washington area jurisdictions that subsidize Metro. So far, money has been authorized for 528 cars, which could begin arriving early next year at a rate of 12 to 16 per month, officials said.

“I hope that before I leave, there will be a 7000-series train running on this system,” Metro General Manager Richard Sarles told reporters Wednesday, shortly after announcing that he will retire in mid-January. Metro spokesman Dan Stessel later said, “We believe the first 7000-series cars will be rolling before the GM hits retirement.”

Stessel was referring to eight 7K cars that were delivered to Metro this year by the manufacturer — Kawasaki Rail Car in Lincoln, Neb. — and have been undergoing tests on Metro’s tracks for months.

Once engineers have certified that the 7000-series cars function properly on Metro’s rail network, those eight test cars will go into service as a passenger-carrying train, and the transit agency will direct Kawasaki to begin mass production, said Joseph Reynolds, Metro’s chief of vehicle program services.

“We have, I would say, another month’s worth of testing,” Reynolds said. “Pretty much that’s it.”

The 7Ks represent a radical departure from Metro’s past.

Over the past four decades, since the Washington area’s underground transit system was born, Metro has ordered 1,134 subway cars in six batches, nearly all of which are still operating. From the 1000 series, built in the 1970s, to the 6000 series, manufactured in the mid-2000s, each batch of cars has largely resembled the one that came before it.

Transit officials wanted each new batch to be mechanically and stylistically compatible with cars already in the fleet, so that cars from different batches could be mixed together in trains. But maintaining compatibility meant limiting the technological and aesthetic upgrades in each new series that rolled off the assembly line.

No so anymore. The 7Ks feature an array of changes, including a different interior layout, new kinds of seats and floors and 21st-century mechanical and digital systems. The new cars are so advanced that they will run as separate trains, never coupled to the older cars, because they are incompatible with the existing fleet.

During the months of tests, specialists from Metro, Kawasaki and the consulting company LTK Engineering Services have encountered some problems with onboard computer software that allows the various electronic systems in a 7K car to communicate with one another, Reynolds said.

“So basically we’ve just been fine-tuning the software to meet the requirements,” he said, adding that engineers have met with no other serious difficulties. “We don’t have, at this stage, any issues with the hardware. It’s just the software. And we expect that within a month, we should definitely have all those issues ironed out.”

When those eight cars, joined as one train, begin carrying paying customers, the train won’t be assigned to any one line, but will run on different lines at different times on as “as-needed” basis, said Rob Troup, Metro’s deputy general manager for operations.

The decision will depend on how many older cars are out of service because of mechanical or other problems on a given day, and where in the rail system those disabled cars are located. “We call it ‘balancing the yards,’ ” Troup said. “If one line is light on cars, and we have an abundance of cars on another line, we’ll move cars from one line to another.”

He said the 7K train “could be on the Red Line one day, and the next day it’s on the Yellow Line or the Green Line.”

Unlike cars in the existing fleet, which can be run in six- or eight-car trains, the 7Ks are designed to be operated only in eight-car configurations. The mass production schedule calls for 56 new cars to be delivered by June. Combined with the eight cars in the test group, that would give Metro 64 new cars, meaning eight 7K trains would be in service next summer.

After that, 300 more cars would arrive by February 2017, followed by an additional 164, arriving at different times, for a total of 528 new cars at an overall cost of $1.46 billion, Metro said. In the same time frame, the oldest cars in the fleet — 300 cars of the 1000 series, dating to the opening of the subway in 1976 — would be retired, having reached the end of the 40-year life spans.

The 1000-series cars were implicated in Metro’s worst disaster, a 2009 Red Line crash in which a moving train struck a stationary one near the Fort Totten station, killing nine. The victims were in a 1000-series car, which was crushed. The National Transportation Safety Board found that the cars, built to 1970s standards, were insufficiently “crashworthy.”

Since then, while awaiting production of the 7Ks, Metro has been placing 1000-series cars in the middle of trains, protected on either end by newer cars that are better designed to handle the force of a crash. The practice, known as “bellying,” has not assuaged the NTSB, which said 1000-series cars, no matter where they are positioned, pose “an unacceptable risk” to riders.

The 528 new cars for which funding has been approved are not all the 7Ks Metro wants.

The transit agency hopes to eventually buy 748 of the cars, to comprise about half its projected rail fleet by 2018. But the money for those additional 220 new cars — $614 million for the rolling stock, plus $856 million for related infrastructure upgrades — is contained in a long-term capital improvement plan that Washington-area jurisdictions have not yet approved.

The plan, called Metro 2025, includes about $7 billion worth of projects, one of which involves the rail cars. Metro 2025, already endorsed by the transit agency’s board of directors, has been under discussion in recent weeks in meetings among transportation officials from Maryland, Virginia and the District.

Meanwhile, the clock is ticking on Metro’s contracted options to purchase those additional 220 cars, in two batches, from Kawasaki. If Metro doesn’t exercise the purchase options by next June 30, they will expire, spokesman Stessel said. But the agency can’t move forward unless the funding is approved in the Metro 2025 plan.

Sarles said he will continue lobbying transportation officials to approve the plan until the day he retires as general manager.

“I’m still here for a few months,” he said Wednesday. “And one of the things I’m working on is gaining the regional support for that funding so we can exercise the options on those cars. I would say so far we’ve gotten good support. But there’s more that needs to be developed.”