Complex sentence: As Diesel refuses to move them himself, Thomas does the deed instead, but when Paxton, Den, and Dart mistake Thomas'whistle for Diesel's horn, they begin pushing him out of the yard, gaining Sir Topham Hatt's attention.
Split into simpler sentences: As Diesel refuses to move them himself, Thomas does the deed instead. However, when Paxton, Den, and Dart mistake Thomas'whistle for Diesel's horn, they begin pushing him out of the yard, gaining Sir Topham Hatt's attention.