What I remember of it: Toxic future Earth. Cities under domes. A train crash exposes a woman to the contaminated outside world, but she sneaks back in, with a stone from "outside" secreted in her pocket. I think she was part of a team of people using their minds to try and construct a psychic whatsamajig, a force that was going to somehow scrub the Earth clean. I suspect the name of the psychic whatsit was the name of the book. Thanks.
This is Gordon Dickson's The Pritcher Mass
The main character, Chaz Sant, was the one exposed to the outside:
[...]the car Chas was in had broken open.
He was outside.
He was exposed to the rot, potentially infected. According to law, neither he nor any of the other commuters in that particular car could be allowed back into a sterile area again.
Oh yes, he would.
The grim refusal to accept what had happened to him exploded instinctively inside him. He was bound for the Pritcher Mass, not doomed to wander a desolated world until he died of starvation or choked on the feathery white fungus growing inside his lungs. In this one case - his own - the inevitable must not be allowed to happen.
He took the rock from his forehead, about to toss it aside - then something stayed his hand. In the flickering light that he now saw come from the burning engine section of the first car, which lay on its side, he looked at the rock; and a word came into his mind.
This was his chance, if he wanted to take it. A Heisenbergian catalyst, reportedly, was most often something just like this. A piece of wood or stone, not different from any other - illegal only because it was from an unsterilized area as this was.