I still vote flex track. I installed a piece around a corner that was giving me grief, lots of derails, etc. I did not solder the track ends to the other pieces at all. I did exactly what a previous poster mentioned, just pushed nails into the cork to hold the track, and once you add ballast and glue, that should be good enough to hold, or you could use latex caulk as well under the track.
It is always nicer not to have the track joints, as that is where the derails are going to occur. And if for some reason your tunnel is longer than 36" you could just solder the 2 pieces together prior to install. I think the hardest part is cutting the ends to line up with the next piece of track. I use a dremel tool with a cutting wheel, that seems to work well. Just cut it a bit long if your concerned, then sand down the rail to fit. Not hard at all, just make sure to wear safety glasses when using the dremel!