A merger of Chessie System Inc. and Seaboard Coast Line Industries Inc., combining two railroad giants in a stock exchange valued at more than $1 billion, was approved unanimously by directors of both companies yesterday.

Chessie and SCL officers said they expect to establish a new holding company, with the tentative name of CSX Corp., which inturn would buy the existing railroad firms in an exchange of stock. CSX would be organized under Virginia law and officials are considering Richmond as a possible headquarters location.

With 27,000 miles of routes in 20 states and Canada, CSX lines would make up the nation's largest rail network. Combined annual revenues of more than $3.2 billion would exceed any other rail firm and rival two airline companies - Trans World Airlines and UAL Inc. - as the country's largest transportation business.

Virginia Gov. John Dalton, who hosted a news conference in Richmond where the definitive merger accord was announced, hailed the plan as "a positive plan to improve rail freight transportation in the East and South, with the public as the principal beneficiary."

Rival railroad executives, some of whose companies could be adversely affected by the business strength of a Chessie-SCL combine, did not want to talk about the situation yesterday. Wall Street analysts were divided in their assessments of the proposed merger's ultimate success.

If the merger is consummated, however, there was general agreement yesterday that other railroad mergers soon would follow.

Shareholders of the two companies will be asked to approve the merger at meetings early next year, after which a formal proposal will be submitted to the Interstate Commerce Commission, the federal agency that must approve such combinations. The Justice Department is expected to conduct an investigation of impossible anticompetitive aspects of the proposal, which is normal in huge merger cases.

At the news conference yesterday, in which reporters from around the nation were invited to participate by a specially arranged telephone conference call, SCL Chairman Prime Osborn III and Chessie Chairman Hays Watkins outlined their accord as follows:

Each current owner of Seaboard stock would receive 1.35 shares of the new holding company for each Seaboard share and each Chessie owner would get one new company share for each share they own, virtually the same ratio forecast by Wall Street analysts in a story published yesterday in The Washington Post.

The Seaboard-Chessie combination would be a 50-50 partnership, with the initial parent company merger followed by gradual integration of policies and administration in the two systems. Each company would provide half of the initial directors and one of two chief officers - with selection of those individuals left to the new board.

Since there are about 14.6 million shares of SCL common and 20 million shares of Chessie outstanding, the proposed exchange of stock would create about 40 million shares in the new company - each one worth about the price of a Chessie share today. At yesterday's stock closing, Chessie was trading at $26.50; with the proposed 1.35 SCL exchange ratio, that would make each current SCL share worth about $35.75 yesterday - but the ultimate price depends on market value at consummation. SCL closed yesterday up 2-5/8 to 30-1/8.

"A major goal . . . is to increase the railroads' share of the transportation market through improved service and improved access to markets for shippers in the north and south," including through service along the rail lines.

Southern Pacific Co., which SCL turned down earlier this year as a potential merger partner, has purchased about 10 percent of SCL stock and, according to Watkins, would become the largest single owner of the new CSX Corp. Chessie owns some 5 percent of SCL.

San Francisco-based Sopac called the Richmond announcement "an interesting development (that) should put to rest once and for all, the charges of Seaboard that Sopac is and has been in an unlawful control position with reference to Seaboard due to its stock ownership."

Sopac Chairman Benjamin Biaggini was informed about the merger in a telephone call from Watkins, but Biaggini had no immediate comment on whether his firm would launch a rival bid for SCL. Reportedly, Sopac was offering to buy SCL for about $35 a share.

In Washington, Southern Railway president Stanley Crane declined to comment on the proposed merger which would open up routes throughout the north to SCL, the chief southeastern competitor of Southern. SCL owns Seaboard Coast Line and Louisville & Nashville while Chessie owns the Chesapeake & Ohio, Baltimore & Ohio and Western Maryland lines.

Southern is studying a possible takeover of the Illinois Central Gulf railroad in the Midwest and expects to make a tentative decision by year's end, company spokesman Charles Morgret said.

Among businesses that would become subsidiaries of the new CSX Corp. are Chessie's extensive timber, coal, and real estate holdings, as well as the Greenbrier resort hotel in White Sulphur Springs, W.Va., and SCL's newspaper operations in Florida (daily papers in Jacksonville and St. Augustine, plus several weeklies).