WASHINGTON'S ARID spring hasn't made groundbreakings all that easy, but the weekend has seen the official start of Green Line subway station construction at 13th and U Streets NW. This is the line that will make an important difference to the poorest neighborhoods of the region and a major difference, too, to the richest suburbs if it succeeds in matching downtown's job-seekers with the openings in the booming outskirts. Even the sharpest critics of the rail system can recognize the significance of this line to the neighborhoods it will serve and to the overall ridership. In the meantime, there are more money matters to attend to -- and fortunately so far Congress is delivering.

A House subcommittee has ignored the Reagan administration's move to cut off all federal money for Metro subway construction, voting instead to provide $217.2 million needed mainly for the Green Line. This proposal for fiscal 1987 is the same as the amount for the current fiscal year -- it's a fair and workable figure. The move enjoyed solid bipartisan support, which it will continue to need through the House and then the Senate. If it is coupled with additional financing from the state and local governments, which have paid willingly and on time all along, completion of the subway project in the capital city can proceed as envisioned over the years since the Eisenhower administration.

In the meantime, the governments of the region and the Reagan administration are said to be nearing an agreement that would end the administration's refusal to release money appropriated for Metro for the last two fiscal years. This, too, can't come too soon. All parties of the region are now looking harder at the costs not only of completing the 103-mile system but also of operating it. These obligations are sobering, and it is essential for Metro's federal-state-local partnership to continue addressing them.