THE GIGANTIC trade deficit has at last created a consensus that something's got to be done. That conviction is now racing through Congress like a freight locomotive, and everybody with freight to be carried -- every lobby, political faction and ideological splinter -- is trying to attach its own boxcar to the train. There are trade protection bills, trade expansion bills and trade reform bills. A few have useful provisions. Most would benefit one industry at a large but hidden cost to other American businesses. All of them claim to make international trade fairer.

But fair trade doesn't have much to do with the trade deficit. They are separate subjects. The distinction is central, although it is in danger of being run down by the freight train. Most people in American politics respond warmly to the familiar and congenial idea of fairness.

"I will not stand by and watch American businesses fail because of unfair trading practices abroad," Mr. Reagan vehemently said this week. The sentiment is repeated by all the people in Congress, most of them Democrats, who support the various protectionist atrocities gathering momentum there. But even where legislation would genuinely make trade fairer, it won't do much about the fact that this country keeps importing far more than it exports.

This country's international current account -- which counts trade in both goods and services -- was running a surplus as recently as the first half of 1982. Japan certainly engages in practices that are unfair, as does the European Community. But they were engaging in the same practices and worse in 1982. If unfairness can't explain the soaring American trade deficit, what can?

It's mainly the dollar's exchange rate. Three years ago it was still trading close to its actual value in terms of things that it can buy. Then the American borrowing binge accelerated, led by the Reagan administration with its grossly unbalanced budgets. To finance all that borrowing, American interest rates sucked in foreign money. That pushed up the exchange rate, and the result is the deluge of inexpensive imports. There were other causes as well -- low growth abroad, the inability of indebted Latin countries to buy American exports -- but they are minor compared with the crushing impact of the high dollar.

The administration, to its credit, is now openly trying to ease the dollar down. But that will be difficult, and it will take time. It will also ultimately require serious reductions in the federal budget deficit, for which neither Mr. Reagan nor his adversaries shows much enthusiasm. That's why you are going to hear a lot this autumn about protecting upstanding American industries against unfair foreigners and perhaps see a lot of bad legislation passed. But if you want to know where the trade deficit is actually going, keep your eye on the exchange rate.