The biggest compliment you can give to a steam train seems to be that they are a “really useful engine”, which backs up your point somewhat. You didn’t mention the fact that the evil diesels are generally black, and the good steam engines generally aren’t. It’s not universal, Donald and Douglas and some other steam engines are black while some bad characters like Diesel 10 and Old Stuck-Up are different colors, but it’s possible to make an argument that black is bad in Sodor.
Coming from the British side, I often found the characters in US TV shows to be too happy and enthusiastic about everything. It was nice to see some counterbalance when Daria came out.
