BURIED in the details of President Reagan's economic program is the outline of a new national transportation policy.If all his proposals become law, the federal government will abandon its role as the promoter of one mode of transportation or another and reduce its effort to curtail gasoline consumption. Whether the administration had so sweeping a change in mind is not clear. But its proposals tilt toward automobiles and trucks and away from trains, airplanes and barges.

The administration's stated goal is to make each mode of transportation self-sufficient. The users of aviation would pay the cost, through higher taxes on tickets, freight and fuel, of operating the airways and building new airports. Those who move freight on the inland waterways would pay for the operation and expansion of that network through higher barge fuel taxes. While new taxes would not be imposed on railway users, the subsidies that have kept passenger trains in operation would be reduced and those that have helped keep marginal freight trains -- many of them operated by Conrail -- alive would be eliminated. In urban areas, the costs of mass transit would be thrown back to the riders or to state and local governments, with the federal government's major assistance coming only in the purchase of additional buses and the completion of rapid rail projects.

The net result of these changes would be to make transportation by air, rail and water more expensive while that by highways would remain as it is. Increase in oil prices will, of course, drive up auto and truck costs, but they also drive up the costs of operating airplanes, trains and barges.

The goal of the Reagan administration, presumably, is to make the federal government neutral as regards transportation and to give travelers and shippers the final say on which modes prosper and which decline. That is a reversal of a policy under which the government fostered the building of railroads, waterways and highways and the opening of the airways as the needs of the nation and the available technology changed.

Maybe this is the way in which the nation should be moving. If it is, the changes ought to be made as part of a unified transportation policy and not as fragmented efforts to make individual government programs self-sustaining. The transportation network is vital to both national defense and economic development. It would be a serious mistake to break it into its separate parts and try to finance each independently without some general understanding of what the network will look like by the end of the century.