What is explained in this excerpt that is not explained in "The Steam Engine" is B. George Stephenson was joined by other inventors to enhance the design of locomotives.
The excerpt specifically mentions the contributions of young James, the son of Mr. James, who invented an improved boiler that Stephenson incorporated into his locomotive designs. This highlights that Stephenson was not working alone in his efforts to enhance locomotive design and that he was open to collaboration with others.
Therefore, the excerpt specifically highlights the collaborative aspect of Stephenson's work with other inventors, particularly young James's contribution to the improved boiler design.