Toby's Tightrope Thomas the Tank Engine & Friends : Season 3 Episode 15

Mavis is told that she is not allowed anywhere near Toby's line, so when the trains get bigger she develops a plan for the trucks to push her there. But Toby accidentally takes those trucks and they decide to push him instead. As Toby speeds down the line, he realises that the bridge ahead is collapsing and he knows that he can't stop in time!