A trolley motorman who failed to obey his traveling instructions caused Sunday's head-on collision of two vintage street cars at the National Capital Trolley Museum in which 19 passengers were injured, Maryland-National Capital park police said yesterday.

But the motorman, Barry Smith of Arlington, said the severity of the crash was worsened by the museum's practice of greasing its rails to reduce wear on trolley wheels. He said in an interview that the grease caused the street car he was operating to skid when he applied brakes while traveling at roughly 10 to 20 miles an hour.

Directors of the nonprofit museum said they would not announce an official cause of the accident until the National Transportation Safety Board finishes its investigation. They denied, however, that grease on the rails worsened the impact.

When the old trolleys were running on the streets of Washington, Smith said, they were equipped with mechanisms that would spray sand on greased rails in front of them, providing friction for emergency stops. He said he urged museum officials several times to equip the trolleys with such devices.

But museum officials said yesterday that the mechanisms are not needed because the trolleys rarely travel faster than 20 miles an hour.

The conflicting statements came a day after from 70 to 100 passengers, many of them children, were jostled or thrown to the floor in what officials said was the first trolley crash in the 18-year existence of the popular museum, located north of Wheaton.

Police spokesman Tim Boyle said 15 persons, including Smith, were treated at area hospitals. Only Smith was admitted. He was in satisfactory condition at Montgomery General Hospital last night with a leg injury.

Boyle said Smith, a volunteer motorman, ferried sightseers along the museum's single, 1.25-mile track, then turned a broad loop at the end of the route and headed back on the track toward the station, where he had begun.

Before beginning the 15-minute trip, however, Smith had been told by the station manager to wait in the loop until the trolley carrying sightseers behind him had also entered the loop, leaving the single track clear, Boyle said.

He said the two trolleys, the only ones operating Sunday, had followed that same process in an earlier trip.

Smith was operating a 1918 model street car, museum officals said. The other trolley, a 1935 model, was driven by another volunteer, identified by police as Larry Glick of Silver Spring.

They rammed head-on at a curve in the track moments after Smith left the loop, Boyle said.

Smith said yesterday that he was told nothing before he left the station. He did not stop in the loop, he said, because he assumed the second trolley would not leave the station until he had returned.

Officer Boyle, however, said witnesses told police that Smith did stop in the loop, long enough to use a museum telephone near the track. Boyle said motormen sometimes use the telephone to call the stationmaster, to make certain the track is clear before leaving the loop.

Whom Smith called, if anyone, is a mystery, Boyle said. The volunteer stationmaster, Robert Flack of Silver Spring, said he did not hear from Smith, according to Boyle. Smith, meanwhile, denied using the telephone.