RIVERSIDE, Calif. -

 A teenage boy was struck and killed by a Metrolink train at a rail crossing in Riverside today.   The accident was reported at 5:58 p.m. at the Madison Street crossing, near Indiana Avenue.  The victim was hit by a train heading from Orange County to San Bernardino, Metrolink's Jeff Lustgarten said  Riverside Fire department paramedics pronounced the boy dead at the scene and called for a chaplain to counsel his relatives. The victim appeared to be in his mid-teens, but his name has not been confirmed, Riverside police Lt. Guy Toussaint said.   The circumstances remained under investigation.   Metrolink was bringing in buses to ferry passengers to the final destination, San Bernardino. That train set was scheduled to serve as Train 334 to Los Angeles tonight, but that trip was canceled and passengers were told to use a later train, Train 336, Lustgarten said. Los Angeles County sheriff's detectives were assuming the investigation, apparently because of the department's contract with Metrolink, Toussaint said.   Seventy-seven people were aboard the train at the time, according to reports from the scene.   Riverside Fire Department Division Chief Mitch Wesche told City News Service pedestrians, bicyclists and motorists must always be alert at rail crossings.   ``I'm not sure what happened here, but too often people go around the crossing arms without realizing that just because one train has passed, all the tracks are clear,'' Wesche said.   ``From a public safety standpoint, keep in mind that there are two sets of tracks. Don't assume that just because one train has gone by, you're clear to cross. Another one could be coming from the other direction and blindside you.''