Richmond-based CSX Corp. announced yesterday a major reorganization of its railroad and natural resources businesses that will include the elimination of 6,700 railroad jobs and a major write-off of track, locomotives and freight cars.

Hays T. Watkins, chairman and chief executive officer of CSX, said the changes were approved "unanimously and enthusiastically" by the board of directors. CSX's major properties include two railroads, the Chessie System (including the old Baltimore & Ohio) and the Seaboard System; a barge company, American Commercial Lines, and an oil and gas company, Texas Gas Resources. Its net income last year was $465 million ($3.15 per share).

The changes include:

*A realignment of CSX into four strategic areas: transportation, technology, energy and properties.

*Reorganization of the management of its railroads into three companies: CSX Distribution Services; CSX Transport and CSX Equipment.

*A 12 percent reduction of railroad employment from 55,000 through the use of an accelerated voluntary separation program. CSX spokesman Ray Bullard said the company hopes to complete the cut of 6,700 people within two years.

*A $954 million write-down of book value, including 1,000 miles of "unproductive track," which CSX would not further identify; 600 "surplus and outdated" locomotives and 41,000 idle freight cars.

Early reaction on Wall Street was favorable. CSX stock closed at $31, up 1 7/8, and analysts praised the management actions.

Henry H. Livingstone, of Kidder Peabody, said, "I think it's a very positive move" and estimated that CSX could show as much as a $50 million benefit in 1986 on flat revenue.

Andras Petery, of Morgan Stanley & Co., said, "To me, the most interesting part is the courage with which they are facing up to railroad problems. They recognize that they have surplus facilities, equipment and employment and have set up an orderly reduction of the plant."

Reduction of plant and employment has been the story of railroading generally in recent years. Since 1979, major roads have cut jobs by 40 percent, from about 500,000 to an estimated 300,000 by the end of this year, according to industry specialists.

The reorganization of CSX railroads, Livingstone said, means that "They'll have all transportation sales and marketing in one corporation; all train operations in another, and all equipment ownership, service and supply in a third. I assume this means they will try and get better fleet management and less parochialism."

One of the management challenges at all the new so-called super-railroads is the rivalry between historic opponents joined together. C&O and B&O were competitors for years and are now part of the Chessie System, and neither of them liked the Seaboard, itself a collection of former competitors.

Watkins said the reorganization of the railroads "would sharpen the focus on shareholder value by increasing control over costs and assets; simplifying decision-making and improving managerial initiative, and placing our people and their skills closer to the customer."