Well, as predicted the rail cracked (in 2 places), so I decided to reinforce both rails before it spreads.
After removing the rail tape I could see the cracks better, but just after grinding I could really see how bad it was. Once you grind the board (took me 3 hours) and remove the all the filling (and there is a LOT of filling) you can see how bad the construction is. The carbon layer is so thin and the foam has a so low density that you can squeeze the rails like a soft ball. And after grinding the board I found a place where there was no carbon fibre at all. It looks like there as a wrinkle on the fibre when it came out of the mould, and this wrinkle got sanded to make the surface even, so where the wrinkle was there's a gap of about 150mm long and 1mm wide where you can see the foam.
For this job I ended up using a 240g carbon fibre that comes in a roll like a tape, 100mm wide. I put the tape on the top of the rails, going around it, and put another 50mm wide tape inside the rails. The result after the epoxy is cured is really strong.
I'm doing the filling again, but using very little epoxy as I used peel ply over the carbon fibre so the surface is almost ready to be painted. I'll upload more pictures when the job is done.
Just one more thing regarding weight; the board was 10.98kg before I started the fix (without the fin), and after the carbon fibre and epoxy it weighs 10.75kg, so you can see how much filling was took out of the board. They could've put another carbon fibre layer like I'm doing now and still have a light board without compromising the structure (NSP website says the board weighs around 11.43kg).