This has been a big thing in the Thomas fandom ever since Hero of the Rails came out people called how unrealistic it was for Thomas, an E2 class tank engine to be able to outrun, a streamlined A4 like Spencer but when I watched it again it actually makes sense. E2 being lighter accelerate faster, and Thomas had a slight head start on Spencer. Spencer being heavier it take him longer to build up, and reach top speed, and the fact that Thomas kept switching tracks, and turning bends, and thats a speed killer for bigger engines we also saw when Thomas switched tracks Spencer had to break which means he had to decelerate, and build back up speed, and Thomas was way ahead of him at that point, we saw on the bridge Spencer now on a straight track was finally catching up to Thomas, but the bridge fell under Spencers weight. It was never that Thomas was faster then Spencer, or the scene is unrealistic, Spencer couldn’t catch Thomas because the track never let him reach or maintain his top speed, while Thomas could keep accelerating efficiently through the changing conditions.