Late in December, if all goes as planned, a convoy of flatbed trucks will arrive in the District from Nebraska carrying four subway cars — four gleaming, stainless-steel conveyances unlike any seen before in a Metro station.

And for a middle-aged, earth-toned transit system anchored to the past in technology and aesthetics, it will be a watershed moment, signaling the beginning of the most significant changes in engineering and industrial design in the subway’s 37-year history.

Attention, passengers: The ’70s are finally dead.

Or at least they’re slowly dying.

“No more brown. No more orange and yellow and circus colors,” Metro General Manager Richard Sarles said. “We’re not only creating a highly reliable car, a technologically advanced car, but we’re also saying to the customers: ‘This is brand new. This is something completely different than you had. This is something exciting.’ ”

Although it won’t happen quickly, Metro envisions the biggest leap in the evolution of its trains since the subway opened, as the agency says goodbye (very gradually) to those familiar, ­chocolate-striped cars of dull aluminum that D.C. area commuters have known since the disco era. The four new cars, the first of 748 due to be delivered by 2018, represent a move into the future that is expected to cost about $2 billion, Metro said.

Improved safety features, better seating, more standing room, digital information displays, enhanced audio systems, a blue interior color scheme and no more stained and soiled carpeting or poles blocking the aisles — in describing it, almost giddily, Metro officials sound a lot like auto dealers. “You’re going to love this car,” Sarles said.

“A clean break with our past,” said Rodrigo Bitar, Metro’s assistant general manager for infrastructure and engineering services.

From the mid-’70s, when Washington’s underground transit system was born, through 2008, the agency acquired six batches of cars (1,134 cars in total, nearly all of which remain in service) as Metro expanded, becoming America’s second-
biggest and second-busiest subway network. If aligned end to end, the cars would stretch for 16 miles, each one resembling the next, regardless of how recently or long ago it was built.

The reason: Over the decades, Metro wanted each new batch of cars to be mechanically and stylistically compatible with the cars already in the fleet so that cars from different batches could be mixed together in trains.

The problem: Maintaining mechanical compatibility meant limiting the technological upgrades in each new batch that came off the assembly line. And keeping the cars stylistically consistent meant generally sticking with an interior layout and visual design conceived when bell bottoms were in and Gerald Ford was president.

“When you’re continually trying to match backward, the further away you get from the original, the more difficult it becomes,” Sarles said.

Until eventually it’s time to unmoor from the past and start anew.

“A fresh page,” said Sarles.

Gradually replacing fleet

The 748 cars in the seventh batch — the 7000 series, as it’s called — will constitute half of Metro’s rail fleet by the time the last one arrives in 2018 and nearly 400 older cars have been retired, Metro spokesman Dan Stessel said. The 7000-series cars will run as separate trains, apart from older cars in the system. And in future decades, as Metro acquires more cars, they will be designed for compatibility with the 7000 series.

Eventually — in the 2040s, probably — the last remaining brown-striped aluminum subway car in the nation’s capital will disappear. Subway cars usually have a 40-year life span, Stessel said, and the newest of the current cars went into service in 2008.

The four cars scheduled to arrive in December — built to custom specifications by Kawasaki Rail Car in Lincoln, Neb. — will be tested for weeks on Metro tracks during off hours to determine if any design tweaks are necessary, Stessel said. Then mass production will begin. The 7000-series cars could be carrying passengers by late next year, and 364 of them are projected to be in service by October 2016.

With the new Silver Line set to open in the near future, the 7000-series cars will fill the need for more trains in the system, Stessel said. And they will allow Metro to retire its oldest batch of cars: the 296 cars of the 1000 series, which date to the 1970s and were implicated in Metro’s deadliest catastrophe, the June 2009 rail crash that killed nine people.

Faulty electronic circuitry in tracks was the cause of the disaster, which occurred near the Fort Totten station. The nine people who died were in a 1000-series car, which was crushed in the collision. The National Transportation Safety Board found that the old 1000-series cars, built to 1970s standards, are insufficiently “crashworthy.”

The NTSB strongly urged Metro to accelerate the retirement of those cars. Bitar, the assistant general manager for infrastructure, said recently that the process of replacing the 1000 series with the 7000 series already was underway in 2009, with Metro reviewing designs, talking and visiting with rail car suppliers, and analyzing costs.

While the agency waits for the 7000-series cars to begin arriving, the 296 cars of the 1000 series remain in service, contrary to the NTSB’s recommendation. To provide a cushion in case of a collision, Bitar said, the old cars are placed in the middle of trains, protected on each end by two or more newer cars better designed to handle the force of a crash.

The practice is known as “bellying,” and the NTSB doesn’t like it. A board report warns that the continued use of the 1000-series cars, even in the middle of trains, “constitutes an unacceptable risk to . . . Metrorail users.”

Transit officials disagree.

“In case of another accident — God forbid, hopefully we’ll never find out — but if there is another accident, the newer vehicles [cushioning the 1000-series cars] have technology that will absorb the vast majority of the impact,” Bitar said.

Still, he and others in the agency are eager to see the 1000-series cars removed from the system, probably within the next few years, Stessel said.

He said they’ll be stripped of salvageable parts and likely sold as scrap metal, becoming the first batch of cars to be retired since Metro’s original subway tunnel opened nearly four decades ago.

“It shows you the age of the system now,” Sarles said. “It is symbolic.”

‘Very cozy, very inviting’

An additional 64 cars of the 7000-series are due to hit the rails by February 2017, followed months later by 100 more. Metro will then retire another old batch of cars, the 100 cars of the chronically malfunctioning 4000 series, dating to early 1990s. The final 220 cars of the 7000 series are scheduled to enter service in 2018.

The floors, sans carpeting, will be smooth and black, with flecks of red, white and blue. The seats, minus those annoying outside armrests, won’t feel nearly as cramped.

The doors won’t be as balky as the ones that often irritate riders today.

And vertical poles will rise to the ceiling from almost every seat back, so passengers won’t have to reach for horizontal overhead bars. Metro learned from focus groups that women and short people particularly dislike stretching upward.

“A very comfortable, very cozy, very inviting transportation system,” Bitar predicted.