I did this a week or 3 ago. I extended some of my turntable whisker tracks in newly available square footage (workbench was moved) "land".
I filled the gap with plain old batroom/kitchen caulk, let it cure, then sprinkled/blended new dirt, then glued down with regular soap/glue/water mix.
The old land is at the loco pilots (I didn't even move them for the process, though I did drape a towel over them during the wet work). Longer locos will occupy these tracks. And that window is going.