WITH ALL THE WORRY about how and when Metro might finish its 101-mile rail system, the prospect of adding another 47 more miles of track won't exactly send people into the streets bellowing choruses of "John Henry." But Rep. Herbert E. Harris II (D-Va.) and other members of Congress from Greater Washington think the possibility of extending the agreed-upon network to serve more neighborhoods should be looked into seriously -- and they are right.

"Looked into" is all they're saying, and that is key since that is the exent of a bill just introduced by Mr. Harris and cosponsored by Rep. Fortney (Pete) Stark (D-Calif.). Their proposal suggests these possible extensions to the subway system: 1) a 10-mile link between Alexandria and Burke, along the old Southern Railway route, 2) a six-mile extension of the Red Line from Shady Grove Road in Montgomery County north to Germantown, 3) a 13-mile exension of the Blue Line in Prince George's County from Addison Road to Bowie via the Capital Centre in Largo, 4) an 18 mile-link along the Dulles Access Road to serve the airport, Reston and Tysons Corner, and 5) expansion in the District, perhaps along H Street NE and NW.

As Rep. Michael D. Barnes (D-Md.) says, "This is going to look less and less like pie in the sky as gasoline hits $2 or $2.50 per gallon." Since we're still at least a decade away from completion of what's already on the boards, even that $2.50 may be low; besides, a more important reason to study the additional service is the growth that has already occured since the Metro system was first being planned more than 20 years ago. Mr. Harris also points out that the extensions could be built relatively inexpensively because they would all be above ground and along existing rights of way.

It makes sense. But a word of caution: the proposal should not be allowed to overshadow the more immediate and critically important effort to keep the Metro regional and state alliance intact through current financial squeezes. There is always a danger that money shortages will breed more suspicion among the various jurisdictions committed to the Metrorail system. So far over the years, a combination of faith, persistence and fiscal Scotch tape has seen Metro through some terrible times -- and even if the first 100 miles are the hardest, there's no time now for battles over what to do when after that.