ON THE WAY-OFF chance that President Reagan has the slightest reservation about his administration's decision to cut off all federal money for Metrorail construction, there's a letter in his mail he might want to check. It's from top officials representing the entire national capital region in their capacity as members of Metro's board of directors. And it's not just a sentimental plea to rescue a pet project for another year. They're talking savings too -- pointing out the economic folly of abandoning critical routes designed to link the capital city's low-income neighborhoods with jobs. They also offer some significant math showing the popularity and relative cost efficiency of the Metro transit system.

"This proposed withdrawal of funding could not come at a worse time," Metro board Chairman Gladys W. Mack writes, "because of its impact on the Green Line. After many years of difficult planning choices and legal challenges, all the decisions are finally made. At long last, we are ready to go ahead with this rail line, which will serve the heaviest concentration of riders in the region." Many of these residents are among the poorest and most transit-dependent people in the region, Mrs. Mack notes. It's more than a matter of equity; as the letter states, cutting off Metro wouldn't represent "a true savings" anyway. It would cause serious economic dislocation in the capital, including fewer jobs and poorer mobility.

A few more numbers for the cost-conscious: in October and November, Metro subway fares covered 68 percent of costs, while rail and bus combined are now topping 50 percent, which Mrs. Mack says is one of the highest revenue-to-cost ratios in the country.

If this impressive math isn't enough for President Reagan, he might consider the long history of federal commitment to Metrorail. After all, Mrs. Mack writes, for more than 30 years Metro has been designed, built and operated as "America's subway," with financing included in every executive branch budget since President Eisenhower's administration. "If the current proposal is allowed to stand, this would be the first time any administration has 'zeroed out' Metrorail."

Members of Congress from both parties recognize the fiscal foolishness of wrecking or stalling completion of the national capital's 103-mile subway system. Is there no way President Reagan can acknowledge that he, too, understands the wastefulness and damage of his budgeteer's penny wisdom?