Hi Rich,
My drawing is 8ft 6 by 7ft 6. So, since I got the width right and I matched both width and height with your drawing, something doesn't add up...!
However, it should be possible to shrink the design by a foot vertically without any drastic losses.
Flat boards are for wusses! But really, it should be possible to use flat boards for all the trackwork except, conveniently, the removable section on the west. The river scene south of the station could be a separate dropped section - not true baseboard.
Edit: You could build a simple level removable section first, to get the basic layout up and running as Jaggzuk suggested, and later replace it with a dropped scenic version with the Taw running under the railway.