I've read it a while ago and as far as I remember, it is about a family travelling through the States by a railcar (I think). And they are the only people left in the world, because others have disappeared. And they disappeared because the family wished for it and the wish came true...? What is the name of this story?

Welcome, Mirta.
The story is "The Vacation". You can find it in The Machineries of Joy, and The Stories of Ray Bradbury.

thank you!