...Outside track (biiiiiiiiig, scorpius digital and 1/24) might be one of my further projects, too.
My thoughts so far:
1.Flippersections should be removable in my opinion, on the other hand they might be custombuild with watertight electrics; rest of mechanichs wont need care about water.
2.sections should be short (about 2m I think because of stowing away and to avoid sunheat expansion; only the sum of the expansion causes problems - if you devide the track into smaller parts it wont happen anything; watch real railroad as example.),
3.slot with some drain-holes here and there to drain off rainwater (longitudinal shape e.g 4mm x 40mm to keep water-surface-effects away)
4.stainless steel braid with UV resistant, elastic and watertight glue
5.and all the rest from boatbuilders suppliers: watertight multilayer-plywood, epoxy-coating and 2-component grey epoxy topcoat: will last for decades (know it from my 50yearold sailboat I once owned)
6. some extra space both sides of the track to keep off grass and plants.
I will follow your thread closely, very interesting stuff.