The children are growing up too fast! You have been a forum friend for a long time now.
You will be happy with the multi-level design.
Perhaps a test run of a diesel to check the grade - jumpers clips for power - make a climb and descent run
Do you think you will be able to acquire the insulation board for the ground/earth? I don't recall how that was before foam board. The construction style is like open grid with risers of L-girder style. I will research or perhaps others can tell you other methods.