THE LINE that started to form Thursday morning on the sidewalk outside the Connecticut Avenue offices of the Civil Aeronautics Board is quite unlike the lines we are all familiar with at airports and ticket offices. This one is not intended for passengers. It is a line, filled by representatives of almost all the airlines, to determine the order in which they will file papers in the scramble for routes - and, of course, for passengers and profits - that will begin when President Carte signs the new deregulation legislation. He's expected to do so Wednesday.

The new law will open up for grabs hundreds of air routes all over the country. In most instances, they will go to the first airline asking for them. In most instances, they involve routes on which there is little or no competition or no regular air service. Other provisions of the law will make it easier for airlines to drop routes they no longer want, to lower or raise their fares, to merge with other airlines, and even to steal routes on which other airlines are now providing limited service.

Only the individual airlines know what documents those 20 or so representatives are waiting to file. But the fact that the line began to form at least seven days before the first document can be filed demonstrates the importance the airlines are placing on this event. The people spending the weekend on Connecticut Avenue may be filing applications that will change the whole route structure of the airline industry almost over night.

So far, all the commentary about what deregulation would mean in terms of routes and air services has been based on theory. The documents to be filed after Mr. Carter signs the new legislation will translate that theory into actions. The result wil be interesting, if not decisive, for efforts to deregulate other industries. If the airlines leap at this opportunity to test themselves in a competitive marketplace, as the line suggests they will, and if passengers come out winners through lower fares and better service, as the theory says they should, then the impetus for Congress to lower the barriers in other highly regulated industries will be great.

Net on the list should be the Interstate Commerce Commission and the trucking industry. If competition is good for the airlines, it ought to be even better for the truckers, who now operate in an environment that fosters trips without cargo. It will be a much harder battle to deregulate the trucks. We look forward to the day when the line of people seeking the opportunities that competition brings is in front of the ICC.