Pelham is the end and beginning of the line for the #6 Lexington Avenue local subway. NYMTA names its trains for its terminal points.
I don’t know anything about NYC subways except what I read in books or see in movies. I’m a country mouse. :) It’s been decades since I read the book and saw the movie. I figured Pelham was the name of a street or station—something like that. I just wasn’t aware it was a word for a physical object, too. Thanks!