Gallery Place-Chinatown Station. (Marvin Joseph/The Washington Post)

Metro is suing its largest union to preserve control over the written disciplinary guidelines it uses to penalize workers violating its policies.

In the lawsuit filed Thursday in U.S. District Court, the transit agency argued that it has always had the “clear and unambiguous” right to craft its own policies on the types of penalties that should be used in response to violations — without union negotiators’ approval.

But last month, an arbitrator said differently: Responding to a complaint from Amalgamated Transit Union Local 689, arbitrator David Vaughn decided that Metro must negotiate with the union to institute a new policy on the criteria managers must use to determine punishments.

Now, Metro is calling that decision “absurd and irrational.”

“The Award should . . . be vacated,” Metro’s attorneys wrote in the complaint, “because it is arbitrary and capricious, is irrational, is not supported by the evidence or by reason, is an abuse of discretion, and/or is not otherwise in accordance with the law.”

Union officials say they have the legal right to demand that disciplinary guidelines be subject to a negotiating process — and that Metro must adhere to the arbitrator’s decision.

“The WMATA Compact created by the United States Congress provides for final and binding arbitration of labor disputes between WMATA management and its employees,” said David Stephen, spokesman for the union.

“If WMATA leaders want to go around the Compact and waste taxpayer money at this financially challenging time with a lawsuit,” Stephen added, “then that is a decision that they must stand by and will be held accountable for.”

The lawsuit is a major test for General Manager Paul J. Wiedefeld, who has vowed since he started his job last November to weed out bad apples in Metro’s ranks. To that end, Wiedefeld hired a new general counsel earlier this year — Patricia Y. Lee, who he said at the time would be tasked with “rebuilding Metro from the inside out.”

The lawsuit is the latest legal battle between Metro and the union. In July, the union sued Metro to force it to rehire a mechanic who falsified a report on a tunnel fan inspection two months before the L’Enfant Plaza smoke incident. A month later, Metro filed a counterclaim, arguing that rehiring the worker would be a “serious risk.”

The battle over discipline rules started in 2014, when Metro officials informed union leaders that they were planning to craft a “disciplinary guidelines matrix,” a formal list of appropriate punishments for certain types of violations — part of an effort to increase “fairness and uniformity.”

In a letter to Metro at the time, ATU Local 689 President Jackie L. Jeter made it clear that she expected the union to be involved in creating the guidelines.

“The Union does not necessarily object to establishing an authority-wide system of equitable discipline, but insists that it be negotiated,” Jeter wrote.

In response, Metro’s director of labor relations responded that the transit agency was happy to discuss any union concerns about the new guidelines — but that the union wouldn’t get a final say.

“WMATA deems this an exercise of management’s rights, and not a subject of bargaining,” the Metro official said.

In its court filing, Metro argued that the union had a right to file a grievance on behalf of any employee they believed was punished “without sufficient cause.” But, Metro’s attorneys continued, the union has no right to approve the original punishment decision — either in individual cases or as it pertained to general management policies.

Last month, arbitrator Vaughn came down on the union’s side, demanding that Metro begin negotiations with the union about the policy no later than Nov. 1.

In the dissenting opinion released with the arbitrator’s decision, Donna L. Gaffney — an advocate on behalf of Metro — warned that the dispute could have disastrous effects on the relationship between the transit agency and the union, “impairing the relationship and blurring what have for 40 years been clear lines defining the role of each party.”

“This Award will not only harm the Labor/Management relationship,” Gaffney wrote. “It might derail it.”