Hi Jonte,
Nope, it's made from MDF. If I could do it again, I'd make sure I did it in one piece!
I'm sure once it's filled as you suggest, the casual observer won't know any different, and it's probably easier to do in sections anyway.
Hope you don't mind my interjection, but I tried something similar on my American layout using grey board which, while being resilient to bending (to a degree, of course), was cheaper than mdf. Mine were designed to be portable (the gaps being hidden by tall buildings - or at least that was the idea), but essentially the same as yours.
Hope you don't mind me sharing.
Keep up the good work!
Regards,
jonte