It likely is overkill, but I'm told everything on the railroad is over-engineered, so I'm just following prototype . Some of this track has been down 20 years with I hope many more ahead, and this is in a basement with some dampness.
BTW, in some places where feeders were impossible, I soldered tinned 20 gauge copper across the rail joints
I also want to mention that in the first portion of the layout, I used 3/4" plywood (been so long, I forgot) and have experienced no difference between it and OSB.