REPORTS FROM the underground show that people are continuing to pile on the subway in record numbers. The daily ridership figure topped 300,000 the other day, and it's all Metro can do to accommodate its new-found friends. Some government and private employers are starting to make changes in working hours to relieve the worst of the rush-hour strains, but popular demands for more and better transit service are not likely to taper off anytime soon. On the contrary, they underscore the importance of completing the entire 101-mile subway system as quickly as possible - and the next stop on Metro's winding financial route is Congress.

Here's where things stand: While the local and state governments are working out their parts of the fiscal bargain, Congress is considering legislation to provide the federal share. So far, House District Committee chairman Ronald V. Dellums (D-Calif.), sub-committee chairman Fortney H. Stark Jr. (D-Calif.) and Rep. Herbert E. Harris II (D-Va.) have put together and won committee approval of a good bill. It would authorize the spending of the necessary amounts to complete the last 40 miles of the subway system; to help retire construction bonds; and to help cover operating costs. The legislation is due to come up on the House floor on July 16, and it is a sound compromise that merits approval.

Those newer House members who will be voting on a Metro bill for the first time may have difficulty understanding the complex financial history of this project, but it is important to recognize the federal commitment to the transit system. The latest bill is not just the bailing out of some local transportation project. The Metro subway system has developed over the years with the backing of the last five presidents, as well as the financial support of local jurisdictions throughtout this region and the governments of Maryland and Virginia. This year, Maryland Gov. Harry R. Hughes and Virginia Gov. John N. Dalton have endorsed congressional legislation to provide the necessary funds.

It makes fiscal sense to act now. Delays in the construction schedule simply increase the total costs. As it stands, even if this bill were enacted and nothing else blocked construction, the system would not be completed until 1989. Even now the Metro subway system already has proven to be an important example of mass transit that is working well at a time when people are seeking energy-saving transportation. Approval of the House committee bill is essential to its continued success.