The young Mexican woman who lay down in front of an Amtrak passenger train in Rockville Sunday, killing herself and her 2-year-old daughter, left behind a note at her fiance's Rockville home, investigators said yesterday.

Police would not disclose the contents of the note but said it indicated purpose in the act of Maribel Gonzales, 23, who perished on the tracks at Randolph Road and Nebel Street, along with her daughter, Pamela Arumada. They said Gonzales apparently walked the two miles between the house where she was staying and the train crossing.

Montgomery County police have said only that Gonzales had been visiting her fiance, whom they have declined to identify and who was the father of the little girl.

"This is something she apparently planned to do," said Officer Derek Baliles, a Montgomery County police spokesman, "planned by leaving a note and going from there."

"What she did was the most personal thing she could do," Baliles added. "She decided to take away his most precious possession."

Assistant State Medical Examiner Dennis Chute ruled yesterday that Gonzales's death was a suicide and that her daughter's death was a homicide. "There was a lot of trauma," he said.

Yesterday, along with shredded yellow police tape and discarded purple medical gloves, other evidence of the tragedy remained on the sunbaked tracks, ties and stony gravel where the pair was struck by the Capitol Limited that roared through Sunday about 3 p.m.

Probably carrying her daughter much of the way, Gonzales appears to have walked from the tiny, ramshackle bungalow at Randolph and Viers Mill roads where her fiance was staying to the at-grade railroad crossing where she waited for the train to come.

No one answered the door at the home yesterday, although two battered air conditioners were running, and most window blinds were drawn.

Next-door neighbor Raul Linares, 26, a native of El Salvador who drives a bus for the Montgomery County school system, said the home appeared to be occupied by two men and two women who moved in three or four weeks ago.

"We don't know them, talking like friends," he said. "We just see each other going by. We don't know them real good."

"It's a disaster," he said, especially for "the little girl. She didn't know anything, where she was going or what she was going to suffer. It's really painful."

Jorge Ribas, president of the Montgomery County Civic Federation, said the Washington area has a lot of transient workers, particularly from Mexico, who often live packed into tiny subdivided homes.

"It looks like her committing suicide and forcing death on her daughter fits the picture of someone severely depressed and dejected and certainly not in a healthy state of mind," he said.

Baliles said it was not clear whether she knew that the train passed the spot every day about 3 p.m. or whether she just went there and waited for one. Witnesses reported that she lingered in the area for some time before the train arrived and that the little girl was wearing a blue dress.

An Amtrak spokesman said that the train tracks there are owned by the CSX Corp. and that the speed limit through the area is 79 mph. The speed limit is set by the Federal Railway Administration, the FRA's Pamela Barry said yesterday, and is determined by the quality of the tracks.

Barry said the at-grade crossing is one of many that remain in Maryland. She said as of 1997, the most recent year for which statistics were available, there were 676 such public crossings in the state, though the federal government has been working to eliminate them.

The Amtrak spokesman, John Wolf, said railroad officials were still studying the train's "event recorder" -- which, like an airplane's black box, registers speed, braking, throttle and horn -- to determine exactly how fast it was going.

Yesterday as the midday heat parched the double set of tracks, a freight train roared through the crossing, its horn blaring, shaking the ground as it passed and leaving a hot, dusty wind in its wake.

CAPTION: Police look over site at Randolph Road and Nebel Street where Maribel Gonzales and daughter Pamela Arumada perished.