The railway clerks union, which crippled most of the nation's rail service in a four-day strike last fall, put machinery in motion yesterday for a possible nationwide rail strike in mid-March.

The general chairmen of the Brotherhood of Railway and Airline Clerks voted to authorize a national walkout after the National Mediation Board declared an impasse in negotiations between BRAC and the railroad industry.

Under complicated procedures of the National Railway Labor Act, the board's action means that the union is free to strike after a 30-day cooling-off period that can be extended another 60 days by presidential order for a total of 90 days.

Technically, the union authorized a strike to start "on or about Jan. 18," but it is considered likely that Carter would invoke his powers to extend the deadline.

The union's move was not directly related to another BRAC dispute that could conceivably produce a strike approaching national proportions in mid-January.

The latter dispute, involving the Virginia-based Norfolk & Western Railway, flared into a nearly national shutdown in late September, prompting appointment of a presidential fact-finding board and a court injunction to force all the strikers back to work.

The board made its report on the new dispute last week, and the injunction is scheduled to expire Jan. 14, at which time BRAC would presumably be free to resume striking.

BRAC President Fred J. Kroll said yesterday he is still studying the report and hopes the dispute can be resolved short of a strike but added pointedly: "If we're pushed to the wall, the members ... know how to react."

On the separate question of the national negotiations, Kroll said a bargaining impasse was reached last week after industry negotators insisted that BRAC accept on a "take-it-or-leave-it" basis all contract provisions that have been negotiated for other rail unions.

Eleven of the 13 unions involved in the national rail negotiations have approved new three-year agreements. Only BRAC, which negotiates for more than 100,000 railway clerks, and a smaller union of train dispatchers have yet to negotiate a new contract.

The rail contracts provide for wage and cost-of-living increases of more than 10 percent a year. This exceeds the Carter administration's 7 percent anti-inflation guideline, but the rail unions are exempt because the pattern for the contracts was established before the guidelines were announced in Octobter.

BRAC wants several modifications in the pattern, including some that would raise the cost of the settlement. Among other things, the union is seeking a "cents-per-hour" wage increase instead of percentage gains, which would mean more money because clerks are generally paid less than other railroad workers.

At a press conference to announce the strike authorization, Kroll said his union isn't "strike-happy" but added, "What alternative do I have ... when the industry tries to shove down my throat something that was negotiated by other unions?"

He said negotiators had been making progress toward some modifications until last week when top industry officials decided to hold fast against any changes in the pattern contract.

however, he said he is hopeful that a contract can be negotiated within the next 30 days, saying he would extend the deadline if an agreement appears within reach. The government, he said, should "stay the hell out."