Wooden train project

This is a project that will need some planning and some external help (probably) before the finished product is ready.
The idea is to make a great gift for my nephew and niece to play with alongsinde their BRIO trains and rails. When I was a kid, my uncle did this for me, and I still remember all the fun I had and how cool it was to have something nobody else had (except for his other nephew, that is).
The plans are a little undefined at the moment, but the rough sketch is as follows:
Make one large plate that can be perfectly connected to the rails, and that the trains can run on just as easily as on the tracks. This will be a station and a relay/interlocking plant where trains can be parked, engines can be switched, and perhaps I can find some more ideas...
So far I know that there will be a switchplate that circulates around beween the different tracks, and that is the only gadget I know will be there (except for a lot of tracks with buffers at the end).

But there are lot of stuff I could add:
- An electric barrier
- A level-crossing barrier (electric, or preassure activated)
- A cargo area with loading cranes that can lift containers (if they still exist in the BRIO system)
- Electical lights of different types
- and surely a lot more that I probably will think about later.

The main concerns at the moment are:
- Weight (will be a large wooden plate, so it will be heavy. How to make is as light as possible?)
- Size (got to make it easy to store, and also make it exactly the right length to fit x tracks)
- Tracks (do I carve my own, or split existing tracks and add them on to the plate somehow)