Commuters ride a train at McPherson Square Metro station in Washington in 2016. (Ricky Carioti/The Washington Post)

THE WASHINGTON region may be on the verge of paralysis if the largest union representing Metro workers, Amalgamated Transit Union Local 689, which represents about two-thirds of the agency’s 12,500 employees, makes good on its threat to carry out a strike. One might think the union was driven to the edge of such a drastic move, which would be the first such labor action to hit Metro in 40 years, by an assault on employee rights — an unauthorized pay cut, unjustified firings or blatant disregard for worker safety.

To the contrary.

The “offense” by Metro management that so incensed the union’s leadership that it might incapacitate the region — and risk the wrath of federal courts, which almost certainly would rule a strike illegal — involves shifting a few dozen janitors from one workplace to another and replacing them with contractors.

Granted, the transit union, like most unions, is no fan of outsourcing, which might erode organized labor’s influence even as it saves money or improves service for taxpayers who ride and subsidize Metro. Still, in this case, the disconnect between cause and possible effect is so improbable, and so extreme, that it underscores a big truth: Metro’s biggest union is also its biggest impediment to improvement and modernization, as well as the most serious threat to its financial future.

The union may not have gotten the memo, but reining in costs is no longer a talking point for Metro; it’s a critical necessity. Under legislation enacted by Maryland and Virginia this year, any annual increase beyond 3 percent in Metro’s operating subsidy from its jurisdictional stakeholders will automatically trigger massive cuts to operating and capital expenses amounting to more than $200 million this year and would deal a severe blow to the system’s prospects.

Already, the union, whose leaders stoke grievances by tossing hyperbolic grenades at management, is playing with fire. Even as an arbitration panel prepares to rule on long-standing contractual disputes involving wages, pensions and other benefits, the union has encouraged two work actions this month, including a late arrival by some 500 workers last week that played havoc with schedules and inconvenienced thousands of riders. It followed that by staging a vote by its members — it’s not clear how many — in which 94 percent authorized leadership to call a strike.

Collective bargaining and arbitration are meant to buy labor peace; the trade-off is that strikes are illegal under Metro’s founding charter. By displaying its contempt for that stricture, the union also shows its disdain for Metro’s passengers, who, union leader Jackie Jeter has said, share the responsibility for the system’s long-term decline.

Her remark reflected the arrogance and over-the-top zeal of a union that has lost touch with the public, with financial reality and with the welfare of a transit system whose prospects are critical for the region as well as its own employees. A strike would be a disaster for Metro, for its workers and for the 1 million passengers they serve daily.