In a major policy decision, the Interstate Commerce Commission yesterday paved the way for the construction of a competitive rail line into Wyoming's coal-rich Powder River Basin.

The commission specifically approved transactions giving the Chicago and North Western Transportation Co. access to coal fields now served only by the Burlington Northern Railroad Co.

"We are convinced that competition is crucial where a valuable natural resource exists in such quantities, where rail service is the only practical method for transporting it, and where the presence of another rail carrier would introduce a competitor into a market which is now a monopoly," the commission said.

The decision, effective in 30 days, allows the Chicago & North Western (CNW) to originate coal shipments in the Basin and run them over a 103-mile line already built by the Burlington Northern from Coal Creek Junction to Shawnee Junction, Wyo. From there, the coal will then move over 45 miles of CNW track that will be rehabilitated to Crandall, Wyo.

From Crandall, the coal will move over 56 miles of new railroad line CNW plans to build into Nebraska. There the trains, through an interchange agreement, will begin moving on Union Pacific Railroad track to the major rail gateways of Omaha and Kansas City, then on to their customers, public utilities.

The 103-mile line originally was to have been a joint venture between BN and CNW. But when CNW failed to come up with its share of the construction money, BN went ahead and built the line alone. It has been in operation over a year. The ICC said yesterday that CNW must pay its share of the construction costs of the line within a year.