Welcome to the Forum PeterJohn.
I am assuming that you will want to lift the board(s) on and off your desk. 59" X 26" is quite a large size to lift up and down. Normally 48" x24" is accepted as a comfortable size.
If you are going to move the boards fairly regularly foam board may not be durable enough. The Club I am a member of is building a modular layout using sections 44 1/2" by 16 3/4" We are using 9mm ply for the top and 12mm ply for the frame.
I did start a thread on my modules ( see:
http://www.railwayforum.net/showthread.php?t=10452 ) but this has been dormant for some time as I had to move unexpectedly last May. I am now in a position to get on with my modules, which form a layout in their own right as well combining with those of other club members. At present I am working on the design and constuction of two corner modules and hope to start reporting again soon.
In the meantime you can have a look at the thread which might give you some ideas. I am working in 00.
Hope this is helpful.
Best wishes,
John H-T.