No, that's right. If I do that, there are some strange artifacts appearing at the outgoing end. Terrain inside out, so to speak. Perhaps with adjacent camera's pointing either way, a through tunnel can be made, but I won't need that. Interesting to find out, though.
I never quite grabbed the meaning of orthographic, as I never needed it. Thanks for explaining, it'll be useful.
The TG plane would have to be quite exactly like the mesh, which is a bit too much work, but I have found another way. I needed concrete entrances anyway, to keep the soil from falling into the tunnels, so I modeled them, and put them into place; looks great.