The detail that introduces the central idea that the two cyclists might be related is found in line (17): "It was the other's turn to stare. 'The deuce you are! Why!--my name is Edward Cruttenden!'"
In the provided passage, the central idea that the two cyclists might be related is introduced in line (17) when one of the characters, upon hearing the name of the other, expresses surprise and exclaims, "The deuce you are! Why!--my name is Edward Cruttenden!"
This statement creates a moment of revelation and realization between the two characters. The fact that both cyclists share the same name, Edward Cruttenden, suggests the possibility of a familial or at least a coincidental connection between them. The use of exclamation and the phrase "The deuce you are!" conveys a sense of astonishment and disbelief, emphasizing the unexpected nature of this revelation.
This detail serves as a pivotal point in the passage, leading to a recognition of the similarity between the two characters. The subsequent lines suggest that they are not merely strangers with the same name but may have a more significant connection. This shared name becomes a starting point for the characters to acknowledge their resemblance and prompts them to consider the need to differentiate themselves before embarking on their journey together.
In summary, the key detail in line (17) establishes the central idea of a potential relationship between the two cyclists based on their shared name, Edward Cruttenden.