In the world of word-smithing, there is an expression: ''round words.'' It's usually used in a derogatory context to suggest that a writer has produced words with little meaning. The words are ''round'' because they don't have the sharp edges needed to get a ''handle'' on them.

Well, in some 290-plus pages, the Surface Transportation Board's proposed rail merger rules have a lot of round words.To summarize the six-month exercise into two sentences, the new rules say: If the STB likes you and your merger, it will be approved. If it doesn't like you or your merger, it will never be approved.

Transportation lawyers who have both supported and opposed recent rail mergers are hard-pressed to find any meat on the bones. There's nothing in the proposal, said one, that would have caused the board to reject any of the recent mergers. Nor is the bar raised to discourage any carriers from merging in the future.

That last is galling to some shippers. The 15-month merger moratorium and rule-making to deal with future mergers was justified last March on the grounds that the next merger of major railroads would trigger a final round of consolidation that would leave the United States with just two major rail systems and the current merger rules and policy were not considered sufficient to deal with that eventuality.

Many shippers - more than 100 - participated enthusiastically in the proceeding, telling the STB how they thought mergers should be handled in the future. In a word, they feel they got stiffed by the board.

There's a lot of verbiage about enhancing competition, but little that any shipper could take to the bank. Nor is there anything about remedying existing competitive inequities created by recent STB-approved mergers.

In fairness to the STB, Congress did not create it merely to balance shipper-carrier arguments or even to provide equity. It is charged with advancing the national rail policy, and in doing so, it correctly may tilt toward carriers. The STB is charged under the Staggers Rail Act of 1980, however, with protecting so-called captive shippers from the abuse of monopoly power by railroads. This, the shippers loudly proclaim, the STB has chosen not to do.

One veteran Washington lawyer says: ''All the new rules do is provide a blueprint for the railroads' lawyers as how to craft their next application, which, historically the agency always has accepted at face value.

''The board in its revised regulations imposed no standards with respect to gateways, bottleneck pricing, competitive access and all the other good things it said it would address. All it required is that the applicants indicate in their filing how they propose to address those issues. Indeed, in a sense the new regulations are a step backward in the quest for enhanced competition, for, after having gotten rid of product and geographic competition in rate proceedings, the STB proposes to reintroduce their consideration in determining whether a proposed railroad merger really is anti-competitive. Give me a break!''

A friend with years of experience observing such matters, comments: ''(STB Chairman Linda J.) Morgan seems positive that the proposed rules address the issue of more rail-to-rail competition for shippers. There are some ominous sounding words about divestiture and other possible steps, but the move to put the onus on the railroads to say how they'd increase competition will keep STB in the umpire's chair for a long time. I liked the part about telling railroads they have to say what penalties they must face if they exaggerate merger benefits. I'd say they have to eat their merger application without any seasoning.''

Railroads, which chose not to comment publicly on the proposed rules - they and others have until Nov. 17 to comment to the board - don't seem particularly upset. Why should they, if the rules delay but do not threaten their future transactions?

Union Pacific, CSX, Norfolk Southern and Canadian Pacific all campaigned for a merger moratorium on the grounds not that the Burlington Northern Santa Fe-Canadian National merger was a bad idea, but that their balance sheets are weak and now is not the time for them to be doing merger deals of their own.

A senior executive of an eastern railroad recently said: ''Upon further reflection, maybe the current merger rules need only minor 'tweaking.' ''

They seem to have gotten what they want.

BNSF and CN charged that the board was protecting its competitors and not competition when it imposed the merger moratorium and began the rule-making process. Saying they couldn't ask their stockholders to wait for more than two years, the two called off their proposed combination in July.

If the final merger rules are anything like the proposed rules, a BNSF-CN merger would appear to be a lead-pipe cinch for approval, just as the end-to-end combination was under the old rules. It will be interesting to see if BNSF and CN reconcile and resubmit their merger after the rules are adopted and the merger moratorium ends next June.

I wonder what argument the Four Amigos and Morgan, their ally, will use then?