Thanks for the advice so far.
A few people have asked for a diagram so here's a rough drawing. The 3 boards on right are already in place as my existing layout.
I'm planning on extending into the rest of the garage making a total size 16x8 approx.
Everything on the bottom and left will be new.
I was thinking the entrance on the bottom to descend under the station on the left with a fiddle yard under the boards at the top and the entrance to fiddle yard at top to again pass under station left and have a fiddle yard under bottom board.