When Holden meets Mrs. Morrow on the train, his state of mind is unstable, he enjoys making a fool of her, by lying about her son Ernest, telling her his name is Rudolph Schmidt, the name of the Janitor at Pencey Prep.
Holden is full of contempt for adults. He feels like a failure, he is jealous of Ernest, the fact that his mother is proud of him. On some level, Holden enjoys talking to Mrs. Morrow, because he can pretend that everything in his life is normal.
After this moment of pretense passes and he remembers that he is in trouble, way over his head trouble,he decides that Mrs. Morrow is deluded about her stupid son. So he enjoys fooling her, laughing at her secretly.
He makes the ultimate escape at this point in the book, fleeing from Pencey and leaving reality behind. The incident on the train is only the first of many instances where Holden pretends to be someone else, manipulates the people around him and acts like a huge phony.
Starting with Mrs. Morrow, as his first victim, Holden engages in angry, behavior, lashing out at others. Holden is very close to a mental breakdown, the cumulative stress of his life has caused him to role play to relieve the pressure that he feels. After his lost weekend in New York, Holden is admitted to a mental hospital
The very same actions that Holden accuses of others he is in fact guilty of himself.