George - My layout is a wreck right now, but I snapped this pic of one of the tunnel portals I hand made from a bit of 1x8 white pine and some basswood:
I painted it with some textured spray-paint, than an over-spray of what I thought looked about right for fresh concrete. The little "LL" Lionel Lines keystone I fashioned from a couple of foam letters I stole from my wife's craft-box. The tunnel-lining is brick-paper from the web, pasted onto cereal box cardboard. It works, but it could stand some weathering.
PD