ATLANTA (AP) — One of the nation’s largest railroads must replace millions of defective wooden railroad ties on its tracks because they’re degrading faster than expected, the company said in a federal lawsuit.

Norfolk Southern Railway blames an Alabama company that produced its railroad ties of failing to use proper protective coating on more than 4.7 million of them, the railroad said in its lawsuit filed this month in U.S. District Court in Alabama.

Instead of using materials that preserve the wood, officials with Boatright Railroad Products Inc. ordered workers to “make them black by whatever means necessary” so they appeared to be properly treated but were not, Norfolk Southern maintains in the lawsuit.

“So long as the railroad ties had the same physical appearance as a tie that had been properly treated, it did not matter to defendants if the substance used to ‘make it black’ was actually a wood preservative at all.”

Motor oil, anti-freeze, paint and other substances which would not effectively preserve the wood were used on the ties instead of creosote — the chemical which should have been used, the company alleges.

Boatright also provided misleading samples to a consultant for the railroad who was checking on the quality of its work, the lawsuit alleges. The Alabama firm’s employees were instructed to take the consultant out hunting at the same time he was to be inspecting railroad ties being treated at the facility, the lawsuit states.

A lawyer who represented Boatright in previous legal matters didn’t immediately respond to a request for comment Monday.

Properly treated rail ties are “crucial” to the railroad’s operations, as untreated ones can “degrade and deteriorate prematurely, thereby jeopardizing the safety and integrity of Norfolk Southern’s rail network and the interstate rail network as a whole,” the company’s complaint states.

The state of the nation’s railroad tracks has come under scrutiny in recent months amid concerns about derailments of lengthy trains hauling oil or ethanol in the Midwest and the South. The National Academies of Sciences recently called for more thorough inspections of freight railroad tracks. More frequent inspections are needed to prevent potentially catastrophic derailments of trains hauling oil and ethanol in which ruptured tank cars can produce giant fireballs, the scientific organization said in a report this month.

Virginia-based Norfolk Southern operates freight trains in more than 20 states in the southern and eastern United States. Several Amtrak routes also use Norfolk Southern’s tracks for passenger trains.

From 2009 to 2014, Boatright provided Norfolk Southern with nearly 5 million railroad ties, and virtually all of them were installed in the railroad’s network, the lawsuit said. About 4.5 million of them are described as cross ties; and 193,000 are switch ties. Also included in the total are about 72,000 inferior bridge ties, according to the suit.

The lawsuit does not specify how they are being replaced or when that might happen. A company representative had no immediate comment.