Despite a tentative contract settlement between the nation's railroads and several unions, the railway clerks union yesterday set in motion the legal machinery for a strike this fall.

Claiming the railroads are refusing to bargain and the government won't free it to strike, the Brotherhood of Railway and Airline Clarks (BRAC) sued in federal court here to force the National Mediation Board to declare a negotiating impasse.

Under the National Railway Labor Act such an order is necessary before the clock can start ticking through a series of colling-off periods totalling about 90 days that must elapse before a rail union is free to strike.

But the NMB, the government agency the mediates railway labor disputes, has refused to declare an impasse. After BRAC filed its suit, the board issued a statement asserting that it has the "sole authority and responsibility - in the public interest - to decide when its mediation efforts have failed."

The U.S. Court of Appeals here upheld that position in 1970 in a dispute involving the International Association of Machinists, giving the board wide latitude to decide when an impasse exists.

At a press conference called to announce filing of the suit, BRAC President Fred Kroll said his union, the second largest of 11 unions currently negotiating with the railroads, is dissatisfied with a tentative agreement reached last week by three of them.

This proposal, which is now in the process of ratification by the United Transportation Union, the Brotherhood of Locomotive Engineers and the Sheet Metal Workers, calls for wage and cost-of-living increases of up to 35 percent over a 39-month period. The exact amount would depend on inflation levels over the next three years.

A fourth union is considering the offer and five more are currently in negotiations over the proposal.

A 35 percent settlement would considerably exceed the Carter administration's goal of keeping the rail increase under 30 percent over three years in the hopes of setting a non-inflationary pattern for the future.

Krol, whose members are more militant and lower paid than many other rail workers, said BRAC wanted a higher overall settlement, with more money in guaranteed wages and less in cost-of-living adjustments. He said his union also wanted a 36-month contract with increases retroactive to Jan. 1, along with other changes not included in the proposed package.

The proposed contract offers guaranteed wage increases of 14 percent with annual inflation adjustments of up to 8 percent, as opposed to the present limit of 6.5 percent. Kroll said he would be satisfied with the 6.5 percent "cap" if more money is put in the wage scales.

Kroll said the railroads have not negotiated with BRAC since early June and cancelled a session scheduled for this week without explanation. The NMB, he said, has been sitting on an impasse petition from the union since May 16. Kroll said he suspected the strategy was to "box us in a corner" but warned it may be risky.

"Our people are becoming frustrated" and the union may have a "time bomb on our hands," said Kroll, acknowledging that wildcat strikes were a possibility.

BRAC, according to Kroll's estimates represents about 120,000 of the more than 400,000 rail employes involved in the current negotiations. Kroll said the average clerk makes $7.04 an hour. Average rail worker earnings were $7.75 an hour as of January, according to government figures.