The nation's severe grain car shortage has eased, but further supply problems are in the offing, the chief of the Department of Agriculture's Office of Transportation said Monday.

I anticipate . . . that we will have continuing difficulties obtaining cars for shippers . . . (for) at least the next 12 months, Martin F. Fitzpatrick Jr. told a special hearing of the House subcommittee investigating shortages.Mr. Fitzpatrick said the worst shortages, which were highlighted in a report released recently by his office, seem to be over at least for now.

That report found shippers were waiting at least two to six weeks for cars in most areas. Some were waiting as long as 14 weeks.

While delays have eased, he said, the next harvest - of winter wheat - will soon be coming in, and there are strong indications that demand quickly will outpace supply again.

Rep. Jim Slattery, D-Kan., told a packed City Council chamber audience here that unless something is done quickly to increase car supply, I'm afraid we'll . . . have grain rotting on the ground before the next harvest is over.

A panel of shippers painted a bleak picture of chronic shortages, long delays and late penalties.

The next panel, of railroad officials, offered a contrasting view, with Burlington Northern Railroad Vice President Alan M. Fitzwater stating flatly,

From the perspective of the buyer or receiver of grain . . . there was not a grain car shortage.

Many witnesses pointed an accusing finger at the Interstate Commerce

Commission for its failure to resolve a long-standing dispute between the railroads and the owners of private cars, which they said seriously exacerbated the problem.

ICC Chairman Heather J. Gradison, who arrived late at the morning session, defended her reasons, saying the ICC had turned the issue back to the parties a year ago, when they reported they were making progress in private talks.

The agency, however, was informed recently that those talks were at an impasse, she said, and the issue has been assigned to an administrative law judge.

In several testy responses to questions from Rep. Slattery, Mrs. Gradison said no time limit had been set on the administrative procedure.

After Rep. Slattery's statement about the possibility of grain rotting on the streets, as happened during the last severe grain car shortages during the 1970s, Mrs. Gradison relented, and said she will sit down with the judge and suggest that a timetable be established.

The prinipal dispute concerned the access that private car owners have to the railroads and the fees the railroads pay to use cars.

These are critical issues because when the supply of cars exceeds demand, at present, the railroads determine whose cars are used to carry the available traffic and what prices are paid for use of the cars.

Private car owners are reluctant to buy additional cars until they are assured they will get what they believe to be a reasonable return on their investment.

On the other hand, railroads have been buying few new grain cars, which usually are 4,000-cubic-foot capacity jumbo covered hopper cars.

Following the grain car shortages of the 1970s, the carriers bought fleets of new cars and found them sitting idle when President Carter embargoed grain exports to the Soviet Union and harvests eventually fell.