An Insterstate Commerse Commission administrative law judge yesterday approved the purchase of the Detroit, Toledo & Ironton Railroad by the Grand Trunk Western Railroad.

The initial decision by ALJ Richard H. Beddow, subject to review by the full ICC also would permit the Grand Trunk to control the Detroit, Toledo and Shore Line Railroad, a small rail with about 60 miles of track.

In approving the Grand Trunk Western proposal, Beddow ruled against the proposed purchase of the DTI by the Baltimore & Ohio Railroad and the Norfolk & Western Railway Co. The N&W and B&O, which is part of the Chessie System, sought authority to jointly control the DTI. The Justice Department had opposed the B&O-N&W application but did not oppose Grand Trunk Western's bid to acquire the line.

Detroit, Toledo & Ironton is a small railroad operating about 600 miles of track in the heart of the Michigan-Ohio industrial region, mainly between Detroit and Cincinnati. It is the principal carrier serving Ford Motor Co's complex in Dearborn and provides important connections with other railroads.

The Grand Trunk operates 900 miles of track in Michigan, Indiana and Illinois, and is affiliated with the Canadian National Railway.

The case is the first merger decision since Congress amended federal rail merger laws in 1976.

In another development, The Chicago & North Western Railroad announced the acquisition of 9.9 percent of the stock of Western Pacific Railroad. North Western said in a disclosure statement that if has "no present intent" to increase its holdings. Formerly a wholly owned subsidiary of Western Pacific Industries, Western Pacific Railroad was spun off as a separate company in April. The company's primary route runs from San Francisco to Salt Lake City where it connects with other roads.