I've just managed to get around to re-testing.
Does the Ultimate terrain add on use a custom terrain.cfg for FSX? You mentioned in the original post that you used your FSX scenery.cfg file.
I've updated the P3D Terrain.CFG file to include the extra stuff from my FSX Terrain.CFG file.
You also mentioned some textures were copied from only the main texture folder? For the type of data which is causing it to crash, this is the folder where the railroad textures for example would be stored by default:
C:\Program Files (x86)\Lockheed Martin\Prepar3D\scenery\World\texture
You might try merging your FSX texture folder from that location with Prepar3D and try again.
I've also done that.
Everything loads if those two BGLs are removed. With them included there is always a "Stackhash" crash when going to many places in Europe (including EGCC, using UK2000 Manchester Extreme scenery).
I have obtained more details by using ProcMon:
RR54713.BGL crashes P3D just after it has read 4096 bytes from file offset 49152.
RTP4712.BGL crashes P3D just after it has read 1546 bytes from file offset 8192.
(All numbers in decimal).
Without those two BGLs, no crashes, as before.