If you want it to never move, use 1/2".
If you want to be able to change the grade slightly, use 3/8"
If you want to be able to easily change grade over a shorter distance, use 1/4"
The thinner the base, the more support will be needed to keep things where you want them.
Standard half inch thick plywood is all you really need, and then you can use Woodland scenics riser starters to meet it, or maybe homesote if it works better for youWhen building HO grades what plywood thickness do you use? Willl 3/8” work?