FOR A NUMBER OF understandable if disturbing reasons, completion of Greater Washington's proposed subway system has become a popular target of certain naysaying academics and second-thinking politicians who now would either drop the project or shorten the planned 100-mile network of routes to the 60 miles already agreed to. So it is all the more significant that a distinguished organization of influential, concerned and coast-conscious business and professional interests - the Federal City Council - has concluded a months-long study of Metro with a strong call for completion of the full 100 miles and for the levy of a regional tax to help pay for it.

This is no routine cheer report from civic boosters. The council's task force poses tough questions and recommends some fundamental changes in the way Metro has functioned, as indicated in experts printed For the Record on this Page today. Though Council President Sol M. Linowitz notes that "we do not claim to be technical experts in transportation matters," he rightly points out that the group's members have had some experience "in making judgments about complex financial and administrative issues."

On that score, the council task force addresses Metro's staggering financial problems squarely by underscoring the need for a new system of financing the local share of costs. The report suggests that this financing take the form of a regionwide tax - a 1 per cent general sales tax, for example - imposed uniformaly and earmarked for Metro. Alternatively, says the report, consideration might be given to raising enough additional income from a new tax to cover not only the local share of operating and capital costs for the already-committed parts of the system, but also the local share of additional construction to complete the network. The study suggests, too, that a 1 per cent tax on wages applied throughout the region would raise "at least this amount."

Members of the task force, which was headed by G. Duane Vieth, a partner in the law firm of Arnold and Porter, acknowledged the political problems involved in coordinating such a tax in this multijurisdictions ought to adopt a system of local taxes that will produce a specific predetermined amount of revenue earmarked for Metro purposes."

The council report also deals with reservations that have been expressed in statements by the Carter administration about the possibility of completing the full subway route system. Though the federal government would like to find cheaper alternatives, such as using more express buses, the study contends that "bus operations cost significantly more than rail to operate because of their labor intensity. As a result, there will not be a proportional saving in local budgets from cutbacks in rail construction . . ." Moreover, says the task force, the substitution of more buses and automobile travel "will tend to produce results with other cost implications."

In the absence of strong federal or local leadership in support of a firm commitment to complete the planned subway system, this welcome effort by the Federal City Council merits particular attention in official circles as well as in the community at large. In the meantime, we agree with the view of Mr. Linowitz that the council must assume an active leadership role in generating the necessary understanding and support for a sound investment in regional public transportation.