Why did the experience have a profound effect on the train on Elie

I think you might be referring to Madame Schaechter. She is in the same train car as Eliezer during the initial deportation to Auschwitz. She had been separated from her husband and two older sons, and she is accompanied by another younger son. She loses her mind during the journey and screams hysterically and repeatedly about a flaming furnace that she apparently sees in the distance. She terrifies the other occupants of the train, and she is repeatedly beaten by young men trying to silence her. Elie is horrified at the treatment of this woman but also had to admit he wanted her to stop screaming.