Leaders of the nation's railroads have agreed as a group to support substantial deregulation of the rail industry through legislation.

A 14-point plan approved by the board of directors of the Association of American Railroads, the industry's primary trade group, calls for a five-year program of significant decontrol over the industry by the Interstate Commerce Commission.

Under the plan, the ICC would lose -- and the railroad would gain -- a significant amount of control over rate-setting, proposed mergers, planned line abandonments and freightcar operations.

Under the AAR proposal, for instance, ICC rate regulation would be limited to the areas in which a railroad was not faced with "effective competition." Railroads would be free to raise or lower freight rates by 20 percent without ICC interference, and without the 30 days' advance notice now required.

Also, the ICC would lose the power to suspend the rate when a shipper protests. The ICC would have the power to hear a complaint aganst a rate only when the complaining party -- say, a shipper -- could show both that there were a lack of effective competition and that the rate changed more than 20 percent.

AAR officials contend the current rate regulations place the railroads at a disadvantage in meeting competition from unregulated truck and barge operations and bar them as well from earning adequate profits to support the necessary extensive capital improvements program.

AAR officials conceded that with reduced regulation, that some rates would go up, but they also contended that some rates would go down as railroads seek to attract new traffic. "We just want to be treated as much as possible like the rest of American enterprise," AAR President William H. Dempsey said.

Unlike most of American industry, however, the railroad leaders decided to seek to retain the powers of regional rate-setting conferences, whose activities in setting rates jointly are granted antitrust immunity by the ICC.

The one area in which the rail leaders couldn't agree was the issue of joint rates. Under present law, the ICC can compel through and joint rates -- the rates collected for freight shipments that move on more than one railroad.

About two-thirds of all freight traffic currently moves over two or more railroads. Dempsey said there is a sharp difference of opinion in the industry over the issue.

Dempsey said the AAR is drafting legislation reflecting the proposed changes but doesn't know whether it will seek to have it introduced or work from the as-yet-unproposed administration deregulation plan.

Dempsey said he believed there is an atmosphere generally hospitable to deregulation and a "keen appreciation" of the railroads' difficulties among many on the Hill. "We've got a system, much of which is held together with baling wire," he said. With deregulation, there is a chance to get a "better system" that in the long run will aid all shippers, he suggested.

Meanwhile, on Capitol Hill, Transportation Secretary Brock Adams defended the administration's request for an additional authorization of $654 million for the trouble-ridden Northeast Corridor high-speed passenger train project.

Rep. James J. Florio (D-N.J.), chairman of the House transportation and commerce subcommittee, opened the hearing by listing "disturbing reports" that "inadequate planning, poor organization, mismanagement, waste and outright fraud" may have been major factors in the excessive delays and escalated costs that have characterized the project since its inception.

Alan S. Boyd, president of Amtrak, was not as sanguine as Adams about the project's funding predictions. "You should not be surprised if I or my successor is back in the future with Mr. Adams or his successor" to ask for more money, he told the subcommittee.

Boyd, who came to Amtrak last year, said there had been waste and inefficiency, "and maybe there has been fraud" in connection with the program, but he said many of the problems had been cleared up.