I guess it's a case of "great minds think alike", or some sort of "carma" I guess.
I kinda like the black, or possibly some darker shade of grey, that would more resemble the inside of the tunnel. I would also add some angled raised pieces above the portal arch, that would sort of "fan" out, further adding to the "stone arch" effect.
My first thought when looking at the design was that the portals were actually highway tunnels rather then train tunnels. Either way, I would add either tracks or a center dividing line starting at the bottom and "fading" out as if to create a sort of perception of depth inside the tunnel, as the line, or tracks appeared to get further away. It would be pretty easy to just paint over the lines if the effect didn't look right.
Whatever you decide Alex, it's going to look great.
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