The original half of my layout is on a hollow-core door. I mounted it using hinges on one side and have access to the wiring by swinging the door on the hinges. I ran all the wires to terminal blocks on the fixed table. The only real problem I have with this is that I continued my layout as fixed in place and now where I join the two, I have problems with aligning the tracks as they cross between the two sections. They line up, but when swing open the door, the tracks never seem to go back where they belong because the table can move ever so slightly. I think I have a solution for that though.
Anyway, if you do choose to run your wires underneath, you will have difficulty feeding the wires through the door. I have a solution for that using soda straws.