Federal investigators have found that an on-train signal system could have given false information about other traffic on the track to the engineer of a ConRall commuter train shortly before it struck an Amtrak train near Seabrook June 9.

Furthermore, six railroads have been warned by the Federal Railway Administration that as many as 400 locomotives or commuter cars nationwide have the faulty signal equipment. The railroads have been advised to change operating rules until it is fixed.

The discovery has grown out of the accident at Seabrook when the ConRail commuter slammed into the rear of Amtrak's Montrealer, another passenger train. Sixty-eight people were injured and damage to the track and equipment was estimated at $325,000.

The National Transportation Safety Board, which advises on an investigates safety issues but has no regulatory power, recommended to the Federal Railway Administration Tuesday that operating instructions on the commuter cars be ordered changed "until this equipment is repaired." The railway administration has the power to order such changes.

According to tests the board has conducted since the two-train collision, a signal in the engineer's cab "could falsely display" traffic information that would permit the train to legally operate as fast as 30 miles an hour under conditions that would call for a 15 mile an hour restriction.

"It appears that this is a design failure and could involve all cab signal equipment of this type," the board said. Railway administration spokesman Joann Sloan said that the six railroads with that type of commuter equipment are the Burlington Northern, the Rock Island, the New Jersey Department of Transportation, a Philadelphia commuter railroad, ConRail and Amtrak.

In a telegram to the railroads the railway administration suggested that a warning notice be placed in each cab and that special instructions be issued immediately. The suggestion is not mandatory, although the agency has the power to order compliance.

"I think we've fot it covered," said Robert H. Wright, acting chief of the railway administration's safety office. The six railroads must send the agency a copy of their special instructions.

It was also learned yesterday that possible brake problems on the ConRail train are a focus of the investigation, although neither the brakes nor the signal equipment have been specifically blamed.

In braking tests, the safety board discovered that if the brakes of the ConRail train "had been applied at the point where the Montrealer was first sighted," the ConRail train "could have stopped short of a collision at any speed less than 60 m.p.h." The ConRail train was going much slower at the time of impact.

The investigation is continuing.