Turns out that the carpet in Metro’s trains, that grimy, orangey-brown, 1970s-era relic that is a source of both nostalgia and derision among riders, has a rather checkered past.

An investigation by the agency’s Office of Inspector General found Metro had “exceedingly stringent” requirements for its rail-car carpet, stretching back decades, that were probably written to favor one supplier.

In fact, the “100 percent pure virgin wool” carpet was made to specifications no longer in use in the industry, the inspector general said, resulting in the “appearance of favoritism” toward the contractor.

The recently concluded investigation found that Metro’s standards for its carpeting were unchanged for two decades and that no other vendor could plausibly compete for the contract.

Moreover, the carpet lacked a required coating to prevent fungus and mildew, Metro Inspector General Geoffrey Cherrington said — although it did meet standards for being fire-resistant and mothproof.

Further investigation found the carpet’s compliance testing was not being performed by an independent facility, as Metro requires, but by a laboratory with ties to the carpet manufacturer.

“The director of the lab used by the vendor is married to the Chief Financial Officer of the company that provided the vendor a line of credit” for the carpet order, according to a synopsis of the investigation included in a report to the Metro board.

Metro suspended orders for the carpet in response to the investigation. The report does not name the vendor, and Cherrington declined to provide the name. It’s unclear under which administration the carpet contract began.

“This is simply a matter of a contract specification that had been carried forward through the years,” Metro spokeswoman Sherri Ly said. “The carpeting meets all required safety standards and there is no need for removal.”

The carpet in Metro cars has long been a source of fascination — and consternation — for riders.

“Metro cars may not be elegantly carpeted, but carpeted they are, much to the astonishment of visitors,” historian Zachary M. Schrag wrote in “The Great Society Subway,” his chronicle of Metro’s history. Even in periods of financial distress, the [Washington Metropolitan Area Transit Authority] board has refused to part with this visible luxury.”

Amid the varying opinions on the look and feel of the Washington area’s Metro system, tourists, daily commuters and new riders alike usually have the same question: What’s the deal with the carpet?

“It feels like an old rustic house,” said consultant Suhail Naber, 30, of Rockville. “Like it’s a cheap old setting.”

The decor is a relic of the car-crazy era when Metro opened, Schrag said.

“All of this goes back to a sort of more basic sense that ‘this is not going to be transportation of the last resort, this is going to be transportation that people use,’ ” said Schrag, a George Mason University history professor. “And if you want to get people out of their cars, you’ve got to have air-conditioning, padded seats, tinted windows. . . . It’s part of a larger package of amenities that were designed to meet middle-class expectations — that these are not simple utilitarian devices for people who have no other choice.”

But over the years, the carpet became known more for the dirt and grime it collected. Riders are especially put off by the way it soaks up liquids — be it rain, slush, spilled beverages or, um, other fluids — and smells.

Metro refurbished some of its 5000- and 6000-series rail cars with noncarpet “resilient” flooring and made the nonstick, confetti-patterned hard floors the standard in the new 7000-series cars. The agency announced a pilot program last year to outfit a dozen 3000- and 6000-series cars with the hard floors and identical padded blue seats, along with vinyl overwraps to mimic the look of the 7000-series cars. The agency does not plan to buy new carpeting for its 5000-series rail cars, which are next in line for retirement.

But carpet remains in many of the agency’s legacy rail cars. Metro bought some previous batches of carpet from Taiwan for $5,200 a car. The most recent supplier was unknown, however. The Washington Post contacted a vendor believed to be the manufacturer, but the company did not respond to requests for comment.

Joe Hadeed, president of Hadeed Carpet, a business that has provided carpet-cleaning services in the Washington region for more than 60 years, said Metro’s choice of a high-quality wool for its rail car interiors isn’t surprising. The 100 percent pure virgin wool Metro bought harks back to an era when manufacturers produced “huge rolls of wool carpet that they sold to casinos and hotels.”

In high-foot-traffic areas, Hadeed said, the new synthetic materials that were cropping up wore out within months.

“A Metrorail car probably had the same issue as a casino — it had so much traffic they would wear out real quick,” he said.

So Metro went with a durable — and more fireproof — carpet befitting airlines and commuter rail services.

At his business, a proper cleaning of a 10-by-75-foot virgin wool carpet would cost up to $525, Hadeed said. That treatment, using $40,000 worth of equipment, is a requirement for sucking out all the dirt and grime that might accumulate — especially on public transit.

Heidi Faller, a federal government employee who lives in Sterling, recalled being impressed by the train interiors when she moved to the region two decades ago. “I remember thinking, ‘Oh, they have carpet? It’s really nice,’ ” she said. “But I think it’s still the same carpet.”

Metro replaces the carpet every five years. The agency has said it is vacuumed weekly and shampooed every two months. Meanwhile, the nonstick flooring Metro has installed is easier to clean and can last for decades.

At least one commuter interviewed on a train with carpet this week appreciated the look and feel of his surroundings.

Sitting in car 5105, a Blue Line train to Largo, 54-year-old Rick Murray compared the carpeted floors and padded seats against the trolley-like rail cars on the Boston “T.” Murray, who works for the federal government, stays in Alexandria during the week, and commutes weekly from Boston, where he lives. “It’s much nicer,” he said of Metro, “like a palace.”

In January, Metro issued a new request for information on carpet for its 2000- and 3000-series rail cars — with relaxed specifications, omitting the requirement of “100% pure virgin wool.”

“Choice of material shall be the Contractor’s responsibility,” provided it meets the other specifications, it said.