I've always felt the rivalry part gets overplayed compared to the friendship. Yeah, they compete, but the real core is how their differences complement each other. Thomas is impulsive and eager; Percy is steady and cautious. It's not about one being better, it's about them covering each other's blind spots. Their rivalry pushes them to be better engines, sure, but the friendship means they're genuinely happy when the other succeeds.
That moment in 'Percy's Promise' where Thomas waits for him instead of rushing ahead for the glory? That's the heart of it. The rivalry is the surface tension kids enjoy, but the friendship is the undercurrent that teaches actual values—loyalty, patience, and appreciating different strengths. It's a very safe, gentle model for kids to understand that someone being your opposite doesn't make them your enemy; you can be rivals in a race and still be the first one to cheer for them at the finish line.
Honestly, I think adults sometimes miss how foundational that dynamic is for young viewers. It’s less about conflict and more about interdependence.