The rhetorical device that is used is Irony.
Irony simply means the opposite of what a person says and this is usually done in order to create a humorous effect.
Based on the excerpt, the author stated that lying at home in bed is the best thing that he's afraid of. This shows that the author believes that lying at home in bed is more dangerous than when he travels by rail.
On the other hand, this statement isn't true but rather travelling is more dangerous than staying at home. Therefore, this is an irony.
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