A new 7000-series train prepares to depart April 14, 2015 from the Pentagon Metro station in Arlington. (Matt McClain/The Washington Post)

Metro’s plan to buy 220 additional state-of-the-art rail cars cleared a major hurdle Thursday as federal officials approved the early retirement of nearly 200 subway cars, even though the federal government has a financial interest in them.

Metro, however, still must pay the federal government $50 million in interest it owes on the older cars.

The new cars, known as the 7000 series, or 7Ks, are far more technologically advanced than Metro’s 1,100-plus older cars. The agency, which wants 748 of the 7Ks, is in the process of buying 528 of them. On Thursday, the Federal Transit Administration removed an obstacle preventing Metro from purchasing the final batch of 220 new cars.

Metro has contract options to purchase the new cars at a set price from the manufacturer, Kawasaki. Because the options expire next month, Metro officials had been anxious over the FTA’s pending decision on the proposal.

“This is good news for Metro riders and we are sincerely grateful to FTA for their timely and comprehensive review,” Metro said in a statement. At its meeting this month, the agency’s board of directors “is expected to consider amending its capital budget” for the next fiscal year to include $431 million to buy the 220 cars.

If the board votes to go ahead with the purchase, it would culminate six years of design work and spending debates involved in bringing a new generation of rail cars to the nation’s second-busiest subway at an overall cost of more than $2 billion.

Metro’s older cars were acquired at different times after 1976, when the rail system opened, and all are compatible with one another, even though many of them are decades apart in age. The 7Ks represent such a departure from the past that they cannot be coupled with the older cars and will run as separate trains.

Metro said it intends to use the final batch of 7K cars to replace the 197 cars of the 5000 series, or 5Ks, which were manufactured in the early 2000s. A subway car is built to last about 40 years, with a midlife overhaul. In about five years, the 5Ks would be due for overhauls, costing nearly $200 million, Metro said. To avoid that expense and to upgrade rail service, the agency wants to scrap the 5Ks many years before their time.

But in the early 2000s, the federal government helped pay for the 5K cars, and Metro still owes the government about $50 million in interest. The transit authority would not have been allowed to retire the cars without the FTA’s permission.

In letters Thursday to Metro Board Chairman Mortimer L. Downey and interim General Manager Jack Requa, the FTA authorized Metro to get rid of the 5Ks but said the agency still must pay the $50 million. The money can be paid “in cash,” the FTA said, or subtracted from federal grants used in buying the 7K cars.

Metro “must honor its contractual obligations tied to the grant funding awarded to the 5000-series rail cars,” the FTA said. The transit authority “is liable and accountable for any and all remaining federal interest.” Metro officials said recently that they anticipated the FTA would not approve the rail-car plan without such financial conditions.

Of the 748 new rail cars to be delivered over next few years, 16 have been acquired. Metro plans to use 300 of the 7Ks to replace its oldest batch of cars, the 1000 series, built in the mid-1970s. And about 100 new cars are to be used as replacements for the mechanically troublesome 4000 series cars, built in the early 1990s.

An additional 128 new 7Ks are needed for fleet expansion because of last summer’s opening of the Silver Line, Metro said. That adds up to 528 new cars, for which funding and production orders have been approved.

As for the remaining 220 cars, if the board approves the purchase, 197 will be used to replace the 5Ks and the rest would be used to reduce the number of six-car trains and increase eight-car trains running during rush hours, officials said.