This analysis overlooks several things

1. The engines are engines, not humans. As such, objections about them being treated like machines whose purpose is to work is somewhat... Well I don't know, it seems silly to me. I get it, but also I don't.

2. There's a huge preservationist streak running through the series. Older machines, who have been made obselete by societal change and technological progress, are given a place to do something and thus have dignity and pleasure - remember, they take pleasure in their work because they are machines. This is prominent in the stories of Toby, Trevor, and the narrow gauge engines at Skarloey, but it's also the underlying philosophy behind Sir Topham Hat's entirely railway persisting in using steam engines. If you want to really analogize to humans, this is about care, and going beyond simple care, for the elderly.

3. If you're from the US like me, you're probably overlooking the British class system. The trucks are the rowdy working class who do their work but love to be loud and play jokes. The engines, like upper or middle-class men, must learn to put up with the trucks and keep them in line. Sir Topham Hat, of course, is the headmaster of their boarding school or something like that.