Consolidated Rail Corp.'s employes offered yesterday to buy the northeastern freight railroad from the federal government for $500 million cash and other considerations they claim would be worth $1.5 billion.

If accepted, the offer would make Conrail the first employe-owned major railroad in the United States.

The bid is doubtless the opening round in a negotiation involving Conrail's 40,000 employes, the Transportation Department's Federal Railroad Administration and, possibly, other would-be purchasers.

Spokesman Thomas Blank said yesterday that the Transportation Department will study the offer carefully and that "we do expect to receive other proposals." Other sources said, however, that the employes are the only potential buyer, although "another one could spring out of the woodwork" as negotiations progress.

Financial analysts who have studied Conrail agree that major concessions from labor will be required regardless of who buys the railroad. The employes' offer, presented at a news conference by an umbrella union organization called the Railway Labor Executives Association (RLEA), includes a promise to extend existing wage deferrals for unionized employes from 1984 to 1987.

That extension will not be available if Conrail is sold to a buyer other than the employes' group, according to Fred A. Hardin, chairman of the RLEA. "Why should we transfer wealth to somebody else?" Hardin asked. About 90 percent of Conrail's employes are unionized; the other employes also have deferred recent wage increases as Conrail sought to improve its bottom line in a period of recession.

DOT is under congressional mandate to sell Conrail, a 15-state, 15,000-mile railroad put together in 1976 from the remains of the Penn Central and six lesser lines. Conrail must be sold in one piece if the railroad continues to show a profit; otherwise, it can be broken into smaller segments and sold, an option the unions want to avoid. Conrail consumed $3.3 billion in federal subsidies between 1976 and 1980, but showed a modest profit in 1981 and again in 1982.

That profit has been achieved through dramatic cuts in jobs (Conrail had 100,000 employes when it started), through the wage deferrals and through exemptions from about $25 million annually in state and local taxes.

The employes made their offer because, "We decided that, in the interests of the railroad, the employes, jobs and rail service to the Northeast, somebody had to take the initiative," Hardin said.

The offer contains these basic elements:

* A total of $500 million cash, to be raised in the financial markets.

* Elimination of all Conrail tax losses, which the RLEA said is the economic equivalent to the government of more than $1.2 billion.

* A "structure which will deal" with Conrail's liabilities for wage deferrals employes have made since 1981 and which the RLEA says will be worth more than $400 million when the present deferral agreement expires in 1984.

A majority of Conrail stock would be held by the employes individually and by their profit-sharing plans, with the details of these arrangements still under discussion. Between 20 percent and 30 percent of Conrail's stock would be sold publicly.

Hardin said that labor fully expects the federal government to require an independent board of directors and management. "We have no problem with Mr. Crane," he said, referring to Conrail Chairman and Chief Executive L. Stanley Crane, who is credited with turning the railroad around.

Conrail's worth has been much debated. Ultimately, it is whatever the government can get, as a New York analyst said yesterday. "Make no mistake," he said, "the employes' proposal is a $500 million leveraged buy-out. It is not a $2 billion offer." CAPTION: Picture, A Conrail train arriving at an auto plant in Lansing, Mich., earlier this year. Employes have bid $1.5 billion to buy the freight line from the government. Conrail photo by Steven L. Lubetkin