How do you make a paper mache tunnel for a train?

I'd start by cutting an old Nestle's Quick box, tennis ball can, Crystal Light can or similar in half lengthwise and securing it to the layout with glue. Be sure it's big enough for your train to run under it.

Mix flour and water into a thick paste. cut strips of newspaper or similar paper, dip it in and start laying it where you want the mountain, right on top of the can.

Do only one layer then let it dry before adding the next layer or you'll experience the joys of mold growth.