Read the chapter “A Very Useful Engine: The Politics of Thomas and Friends" by Shauna Wilton from the book The Politics of Popular Culture: Negotiating Power, Identity, and Place.
TL;DR: The show creates a discriminatory hierarchy between diesel and steam engines wherein the former are subservient to the latter, among other hierarchical traits (diagram from the chapter in question):
Diesel and steam engines are class or ethnic stand-ins, representing the poor and rich/middle-class or non-white and white respectively. The naked discrimination perpetuated by the steam engines can be subtly absorbed by young minds to create an environment in which they feel comfortable discriminating against those viewed as the "lesser class."
Furthermore, the authority of Sir Topham Hatt and other higher-ups is unquestionable and the retribution for refusing to adhere perfectly to their demands is extremely harsh -- the example used by the New Yorker article under discussion is Smudger being made unable to move for a minor behavioral infraction. This is emblematic of a bourgeois capitalist belief system in which workers are mere tools for managers, nothing more.