FOR 15 MONTHS ending last summer — and possibly a good deal longer than that — a pair of employees in Metro’s planning department went on an unauthorized, ethically unhinged spending binge, courtesy of the transit agency and the farepayers and taxpayers who support it. Wielding their Metro-issued credit cards like teenagers set loose in a Best Buy, they scooped up Kindle e-readers, camcorders, designer earbuds, digital photo frames and a BlackBerry, not to mention gift cards, gift baskets, boxes of chocolates, a pair of reading glasses and a picnic tote bag.

By the time they were finished — or rather, play was halted by the intervention of Metro officials — they had racked up almost $10,000 in questionable and unauthorized purchases, many of which were fraudulently reported, according to Metro’s Office of Inspector General. In one instance cited by The Post’s Dana Hedgpeth, who received a copy of the inspector general’s report, an employee submitted inaccurate descriptions for 55 separate purchases.

In the context of Metro’s $1.5 billion budget, the shenanigans in the planning department — with 49 employees, a very small fief by the agency’s standards — are hardly bank-breaking. The offending employees are no longer on Metro’s payroll, officials said. The misspent money was reimbursed to the agency. And, according to a statement from the agency, Metro is “conducting training and instruction, as well as establishing further controls,” to crack down on credit card misuse.

But that’s not quite the end of the story. What’s most disturbing about the IG’s report is that it makes clear that, although just two of the department’s employees were abusing their Metro credit cards, many or most of their colleagues were well aware of it and did nothing.

When the inspector general’s office took its findings to the U.S. Attorney’s Office in the District, prosecutors declined to take the case. They did so, they said, for fear that the “culture of purchase card abuse . . . by supervisors” in the planning department was so pervasive that finding reliable witnesses would be all but impossible. “They all have something to hide, so credibility is a real issue,” prosecutors told Metro.

It would be nice to think that the culture of easy spending for personal items on Metro’s dime was isolated to one department at the transit agency. But what basis is there to think that? Metro’s protocols and procedures governing credit card use are standard across all departments, and training and instruction for authorized credit card users have been in place for some time.

Metro appears to be taking appropriate measures. It is considering reducing the nearly 300 credit cards now in the hands of agency employees and tightening the types of items that can be charged to the cards from approved vendors. It is also moving to ensure that supervisors examine card expense statements in a timely way.

No doubt, most Metro employees and supervisors are honest. But by applying the Reagan Rule — trust but verify — Metro may be better positioned to avoid further embarrassments.