Complex sentence: Sir Topham Hatt arrives and tells Thomas to learn about freight cars and reminds him to keep him in the shed if Thomas is not prepared to learn and orders him and Edward to stay in the yard until Thomas knows enough about freight cars as Edward does.
Split into simpler sentences: Sir Topham Hatt arrives and tells Thomas to learn about freight cars and suggests that he should keep him in the shed if Thomas is not prepared to learn. He then orders Thomas and Edward to stay in the yard until Thomas knows enough about freight cars as Edward does.