Here is how I fill in the space, which is really all your terrain. The strips of plywood are part of a method of sub-roadbed construction called 'cookie cutter' because you cut out the plywood in shapes that you need. Another method, which I used only once on my last layout, is called spline roadbed. You rip a sheet of masonite or 1/4" MDF into 5/16" strips as long as the sheet is long, and then you erect 1X2 risers to support the roadbed, they being fastened with screws to the cross-members or joists of your framing. You turn five or six strips on their sides and glue them together, but only three ply at a time so that you can bend them easily around a screw driven into the top of the risers and then clamp them with about 10 clamps. When they dry, you widen the roadbed with another two or three glued strips glued to them and clamped.
Anyway, note how I do my terrain: swatches of aluminium window screen hot-glued into the shape I want...the contours. How to achieve contours? Stuff soft things, maybe plastic shopping bags stuffed with crumpled newspapers and flyers, in behind the screen to force it into the contours you want. Over that, you slather a ground goop of plaster and vermiculite and Portland Cement. It hardens about 1" thick if I have done it right.
Covered with the goop (camera is now turned about 90 deg to the right from the previous view):
You really ought to consider adding the ground foam, which must be glued into place (I spray yellow wood glue, diluted, to which a drop or two of liquid dish soap has been added) while the goop is still drying. Once it is wet, being plaster based, it will absorb a ton of sprayed glue...so do it within about six to 20 hours.