Maybe a low edge to keep the wheels in place.Welcome to the forum.
You are a brave man having your track close to the pond? Maybe you should install a retaining fence in case of any derailment.
But nicely done....
Ah, the layout is U-shaped, but the track is a single track dog bone shaped like a U.Welcome Graffen. Very nice setting for your railway. You say you have gone for a U shape layout. In general it's good to have a loop/circuit or reverse loops if you can so you can sit and watch trains.