The Gregarious Railfan
RR Station Architecture
Station design evolulion by era.
Station design differences due to geography.
Station design differences due to local availability of building materials.
Station design differences due to RR company preference or financial strength.
Other...
Station design differences due to land availability.
Urban station design.
Rural station design.
Station design differences due to rail traffic density
Links
Other...
.
Railroad/Train Stations And Depots - American Rails with a great explanation of various architecture   
The Railroad Station: An Architectural History, book on station architecture
North Creek Depot Museum  with a bit about station architecture
Todd Sestero's Station lists by state at Railfan Guides of the U.S.     
Pictorial examples