Tunnel Walls

This "Endless Tunnel" article by Dick Hunter might be of interest.

http://www.nmra.org/member/content/endless-tunnel

I've lined my tunnel walls with brick; it looks great.  Go to this web site http://paperbrick.co.uk/ and you can generate any type brick and pattern you want then just print it onto cardstock.

Woodlands Scenic makes and HO tunnel liner form that with some modifications will work as an O-scale liner.  Click on the underline phrase to link.   The form is for a double tunnel, that works well of O-scale.  You will have to add material for O-scale height.  Note the pictures.

If the portals aren't too prominent, the standard "quick-and-dirty" approach is black-painted crumpled-up tinfoil.  You only have to do it where there are sight-lines.

You can always try the Scenery Express tunnel materials, they have Portals, Walls and even flexible wall sections in cut rock, Pennsy rock, and timber.

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I used an  expansion joint for concrete for my walls.

Pieces and parts.  The plaster of paris parts can be modified (Cut, file, sand) to make what you want.  I also had a wood tunnel structure that made it easy to glue pieces together.   The curved tunnel was also a challenge.

I made mine to match the portals. I used strip wood, coated with joint compound, then with grooves cut in every 1":

And, then there are timber portals:

In addition to the great ideas above, I have used light bulb packing material (free). I paint it grey. The two attached photos are for demonstration purposes, but you get my drift.