I've been working a quick method to build a curved tunnel....
I took a shipping box, cut it, keeping most of it's length so that it would form a box (upside down U) over the tunnel track, plus an additional 1" on each side for tabs (think of a U with little tabs sticking out horizontally at the top, then turn it upside down to become a tunnel). I added vertical cuts up one sides of the U and across the roof, repeating the cut every 3 inches, so that the the cardboard could be bent into a "smooth curve". I then pinned that down on the layout to position it, tested clearances with articulated engines and long 21" passenger cars, then hot glued a thin long strip of cardboard along the length of the outer curve so the tunnel would retain it's shape. After that dried, i added a sheet of plaster cloth across the top and the side with the open slits (ie, the outer curve) for some added rigidity. When that dried, I removed the pins from tabs, lifted the tunnel off the track and added my decorated foil lining. I used 2 sheets of decorated foil running the length of the tunnel section, one for each wall and 1/2 the ceiling of the tunnel. I laid the foil into the tunnel, hot glued the foil to the tabs/bottom edge of the tunnel, then ran hot glue down the center line of the top of the tunnel, glued the other edge of the foil to that, then pressed the foil in to follow the walls and curve to the top of the tunnel. I then used screws to anchor the tunnel to the layout (the track would remain accessible later. Tested it with TMCC/Legacy engines.
The results are shown in the vid below taken with my lionel caboose..
btw, the tunnel lining is nothing more than crumpled foil, a quick spray with some gray primer, a few streaks of some stone textured spray paint and then a dry sponging with some white paint for the rock veins.