POOR MR. CARTER. First he got lambasted for trying to prune Amtrak and giving short shrift to mass transit. Then he took the Metroliner to Baltimore - partly, he said, "to show my support for a strong mass transportation system." And what thanks did he get? An American Public Transit Association official called the gesture "a bit paradoxical and inconsistent." And the American Bus Association complained that if Mr. Carter really wanted to boost economical, energy-efficient intercity travel, he should have taken a bus.

Good grief. Such fuss about a two-hour jaunt. Granted, both groups have real concerns about the direction of national transportation policy. And, granted, Mr. Carter invited carping by using the train to make a statement of his own. But if his ride really meant anything, it was simply that he has come to realize that mass transit has a large and growing constituency, and it's good to be seen riding one of the country's really successful trains. If interest groups insist on reading more than that into each presidential trip, you have to wonder what will be forthcoming from the rail buffs, the bus comapnies, the airlines, the auto industry, the powerboat makers, the canoeists and the newly important roller-skate crowd when the Carters take their trip from St. Paul to St. Louis on the steamboat Delta Queen.