The interstate Commerce Commission adopted previously proposed policy guidelines yesterday designed to encourage railroad consolidations.

When considering rail mergers, the ICC will seek to preserve the "essential" rail services of acquired roads or of competing lines affected by a consolidation of two other railroads, the policy statement emphasized. But it said the ICC won't necessarily try to preserve the entire system of financially struggling railroads.

The ICC said it will insist that all or part of a marginal railroad system be included in the merger of stronger roads only when there isn't any other way to preserve the marginal road's service, when the inclusion fits into the merged company's operation and when it doesn't threaten the merging roads' finances.

In the past, the agency has been criticized for complicating rail mergers unduly by forcing inclusion of a separate marginal road in new systems.

The ICC also said it will stress operating efficiencies, elimination of redundant facilities and maintenance of "effective competition" among railroads and other forms of transportation in considering rail mergers.

In another action, the ICC eased rules for truckers hauling goods to and from Canada.

The agency said it no longer will restrict the authority of U.S. motor carriers to specific Canadian destinations or origins. It also will stop setting entry points into Canada that truckers must use and no longer will require applicants to have permission from Canadian authorities before the ICC approves any proposed service to Canadian points.

Licenses will say what points can be served in the U.S., but unlimited authority will be allowed in Canada. Canadian authorities still may limit those rights but they often have been more liberal than the ICC, officials said.

In another development, a bid to extend the rate war in the Alaska shipping business to perishable goods has been rejected by the ICC, according to a spokesman for a company opposing the move.

Leonard Shapiro, a vice president of Totem Ocean Trailer Express, said yesterday the ICC turned down a proposal by Sea-Land Service Inc. for major rate reductions on large shipments of such items as milk, meat, vegetables and fruit. One of the reductions proposed would have cut the shipping price on milk from Seattle to Fairbanks by 25 percent.

Shapiro said the lower rates would have applied only to shipments of 600,000 pounds or more.