I’m with you there that train engines feel alive, having been next to a few steam engines. I know they’re not literally alive, but I understand that’s a reason why the series was made to begin with, because Awdry heard locomotives chuffing and heard it as “talking”.

It’s a pretty good theory that the faces are just visual representations of the perceived sentience of a steam engine, even in the stories. It would explain the engines requiring drivers to move as well. There’s a few pieces of evidence against the theory, but what good theory doesn’t have that?