President Carter yesterday ordered striking employes of the Rock Island railroad back to work and promised further measures to "get the trains rolling again." He cited the urgent need to assure rail transportation for the nation's fall grain harvest.

The president acted after it became clear in the last few days that a continuation of the labor dispute could cause massive economic losses in the midwest once the corn, milo and soybean harvest hits full swing in the next two weeks.

The Rock Island's 7,000-mile system carries 10 percent of all midwest grain and serves 1,650 grain elevators.

The strike has contributed to the worst transportation snarls in years. Western flour mills are operating at half their capacity and 5,000 badly needed rail cars are stranded on the Rock Island tracks.

Carter said he was taking "strong actions" because "not only our farmers, but also the consumers of grain -- here within our own country and indeed throughout the world -- simply must be protected."

The principal action taken by the president was creation of a three-member emergency board, a fact-finding and mediation body that supervises a mandatory, 60-day back-to-work and cooling-off period that began at midnight. Some 1,800 members of the Brotherhood of Railway and Airline Clerks struck the line Aug. 28, demanding a pay increase retroactive to Jan. 1, 1978.

However, the president's goal in acting -- to restore service in the grain belt -- is complicated by the fact that the Rock Island currently is bankrupt and may not have the $25 million to $30 million in cash needed to restart operations.

Carter said he has, therefore, asked the Interstate Commerce Commission to direct other railroad companies to provide service on the Rock Island tracks.

Under the emergency provisions of the Railway Labor Act invoked yesterday by Carter, strikers return to work under prestrike pay and work conditions.

The presidential action drew an angry response from Fred Knoll, president of the railway clerks union. The Associated Press quoted Kroll as describing the move as "nothing but a strike-breaking action," raising the possibility that the union might attempt to defy the order.

Kroll has said repeatedly there was "no way" his union would return without a pay increase. Other railroads agreed to a national pay raise earlier.

Top administration officials said yesterday that if the Rock Island is unable to resume service on its own, the potentially volatile pay issue could become moot. Transportation Secretary Neil Goldschmidt has recommended strongly that railroads picked by the ICC to provide substitute service on Rock Island tracks pay the prevailing national wage.

If the ICC directs other railroads to provide service, the federal government would be obligated to pay for losses suffered by these companies as a result of the ICC order.

The United Transportation Union, which also is on strike, indicated yesterday that it will comply with the back-to-work order. The UTU, representing train crews, joined the strike after BRAC'S members had walked out.

In announcing the emergency steps yesterday, the president called on Congress to move quickly on railroad deregulation proposals that he said would "break the strangulation of out-moded, unnecessary economic regulations." Carter said elimination of controls would help the industry generate revenue to upgrade its services.

An administration deregulation bill sent to Congress currently is being rewritten extensively in the Senate Commerce Committee. Aides said a draft bill would be ready shortly.

Senate sources said a deregulation bill has "high priority" but that the administration measure was unacceptable. They said the Senate bill would contain more protections for railroad customers who depend on only one railroad for service, and also would make substantial changes in the rate-making schemes sought by the administration.

Named to the emergency board were chairman James J. Reynolds, a secretary of labor in the Johnson administration; Nicholas H. Zumas, a Washington attorney; and Ida Klaus, a New York labor arbitrator.