If there is one thing about of model railways which defies description it is the geometry of a reverse curve on a changing gradient. This one gets off to a difficult start because there is a turnout on the gradient (to the left of the camera), the turnout is flat and there is less super-elevation than there might be. Everything flattens out at the location of the tunnel portal.
And so, the tunnel portal is glued down with a 3 mm shim under one side of it. This adjustment makes my gauging vehicle (a 75 ft Mk 3 coach) appear to be upright when it goes through the tunnel. The portal is very tight - about 1/16 inch to spare each side of the coach. I think this is one reason why my layouts seem to take ages to build - I am always trying to get things "on the limit" and I doubt if another 1/16 inch each side would have spoiled the look. The incentive here was to make the portal small enough to look ok with HO scale trains.
The tunnel portal itself is 5mm foam board, the surface is the Scalescenes 'concrete' texture. I drew the quoins in Visio and printed them onto cream paper.
I am trying to make flexible scenery. So in the photo here there are some very rigid pieces of plaster glued onto a fascia panel which can move backwards and forwards a little. The gaps in the landscape will be filled with paper mache on top of aluminium mesh, the idea is the whole structure will have enough 'give' in the right places to avoid cracking.