A man is returning from Switzerland by train. If he had been in a non-smoking car he would have died.
The man used to be blind — he's returning from an eye operation which restored his sight. He spent all his money on the operation, so when the train (which had no internal lighting) goes through a tunnel, he thinks he's gone blind again and decides to kill himself. But before he could do it, he saw the light of the cigarettes people were smoking and realized he could still see.