Post is related to Rail. Railroad is derived from Rail. Railroad is related to Old. Old is related to Steam. 
Ridiculous is a synonym of Odd. Odd is the opposite of Usual. 
Post <--RelatedTo--> Rail <--DerivedFrom-- Railroad <--RelatedTo--> Old <--RelatedTo--> Steam 
Ridiculous <--Synonym--> Odd <--Antonym--> Usual 