Santa Fe Industries Inc. and the Southern Pacific Co. fueled speculation about an imminent merger or other combination of their railroads into a western giant yesterday when they requested a delay in trading their stock on the New York Exchange.

Both firms are expected to make announcements today, industry sources said, but it was not clear precisely what they would say.

A combination of the Santa Fe and the Southern Pacific would complete what many analysts regard as a logical realignment of western railroads. Taken together, the two roads are of about the same size and potential economic strength as the Burlington Northern and the Union Pacific, the other two major roads in the West and themselves the products of recent mergers.

Making the speculation more interesting is the fact the Santa Fe has expressed interest in Conrail, the federally owned northeastern road that the government is trying to sell. It was possible yesterday to draw a scenario in which a merged Santa Fe-Southern Pacific road would acquire Conrail and create the first transcontinental system. The only firm offer to the government for Conrail has come from its employes, through the Railway Labor Executives Association (RLEA).

Robert E. Gehrt, assistant vice president for public relations at Santa Fe Industries, declined comment on whether a merger or combination was an immediate prospect. "We will have an announcement," he said. As for Conrail, he said, "We've been talking to Conrail as recently as last week; whether our announcement will have anything about that, I'm not prepared to say."

Last year, Santa Fe had freight revenues of $2 billion and Southern Pacific of $1.93 billion. Burlington Northern's was $3.7 billion and Union Pacific's (including Missouri Pacific and Western Pacific) was $3.5 billion.

Santa Fe Industries chairman John J. Schmidt signaled his interest in Conrail in a presentation for New York analysts in July. Schmidt said, "The next plan for Santa Fe is to do something about increasing the size and the market power of the railroad . . . "

Analysts were as much in the dark as anybody yesterday as to precisely what possible combination or merger would result. Both the Santa Fe and Southern Pacific have large holdings outside the railroad business.

Last year, Southern Pacific earned $120.1 million, or $2.16 a share, on revenue of $3.1 billion. Santa Fe recorded 1982 profits of $180.2 million, or $2.08 a share, on revenue of $3.2 billion. Santa Fe's stock was selling for $34.50 a share at the close of business Friday; Southern Pacific's was at $40.

Santa Fe's debt-equity ratio is 25-75, according to figures from the Association of American Railroads, while Southern Pacific's is 42-58.