WHEN IT'S TIME to ante up for subway construction in these days of shriveled local budgets, the participating jurisdictions aren't exactly heaping on those Metrodollars indiscriminately. Political as well as financial pressures have forced these governments to squawk and squirm a little more than they used to when Metro's staff presented its spending proposals. That's understandable enough, for this delicate and expensive regional venture can always use a careful financial review in search of additional economies. But now Arlington County officials want to take things a selfish step too far -- by backing out of already-agreed-upon and necessary spending for subway construction.

The county government is threatening to withhold its share of future subway construction costs -- and not because it dislikes the subway. On the contrary, Arlington wants out because it already has its piece of the shared action. All of Arlington's subway stations are built and in operation. So even though past Arlington representatives to Metro joined in approving additions to the system that would be paid for by all -- such as parking spaces and station entrances -- the county is claiming now that it never agreed to kick in.

That's no way to run a regional railroad. As Francis B. Francois of Prince George's county notes, there is the philosophy of the whole Metro agreement to consider: "We can't have one jurisdiction picking and choosing and saying they will fund this but they won't fund that. It's interesting Arlington never raised the issue until they got theirs."

Interesting -- and menacing. Not only does this kind of action threaten the entire regional transportation effort but it isn't even in the interest of Arlington; improvements such as additional parking at outlying subway stations are bound to benefit the closer-in areas.That's why they should not be treated provincially by any of the Metro board members, or the staffs of the participating governments. Just as the subway tracks won't end at the political boundaries, the benefits of an efficient system -- and the spending for them -- are to be shared throughout the area to be served.