IF YOU HAVE EVER called Metro to ask, you may know already that one of the most frequent questions put by the public--and one with perhaps the most frequently changing answer--is: "When will the subway line to (fill in your favorite blank) be opened?" Now, it isn't that Metro has no schedule; it is the cold fact that when it comes to opening new lines and stations, money is the root of all groundbreakings.

There is fresh bad news. Yesterday--only hours after Carmen Turner was sworn in as Metro's new general manager--the Senate Appropriations Committee voted to take a large amount of money out of the total approved by the full House for Metro's rail construction. According to Metro's budget and construction experts, this move--were it to stick--would mean:

* a delay of a year in the opening of the Red Line between Silver Spring and Wheaton.

* a delay in opening the Green Line from Anacostia to U Street of--can you stand it?--11/2 years.

* a delay of six months in the opening of the Orange Line between Ballston and Vienna.

True, any difference between what the Senate may approve and what the House already has approved would go to a joint conference for resolution--whatever that may turn out to mean. But as this region's bipartisan delegations in the House and Senate hasten to point out, the Senate has a chance on the floor to avoid making an ultimately expensive mistake: because of inflation and certain contracting provisions that would have to be revised, what may look like a Senate cost cut may, in fact, be a costly cut

So there is alarm--and it is justified. Every president since Dwight Eisenhower has recognized and encouraged completion of the subway system as an essential, practical element of a balanced transportation system for the area. The Reagan administration, through the actions of two sensitive secretaries of transportation--Drew Lewis and Elizabeth Hanford Dole--has underscored this time-honored commitment. Thoughtful senators also can see that construction is at a point where wise money goes for completion, not delay.