Installation

In Blender open Edit, Preferences, Get Extensions, then use Install from Disk and pick the tunnel_portal_generator zip.

Usage

1. Open the Tunnel tab in the 3D viewport sidebar (N). With nothing selected, click the picture or browse the five families to pick one of the 45 designs, then press Add Tunnel Portal. You can also use Shift+A, Mesh, Tunnel Portal.
2. The card at the top shows the design, the number of openings, the size, face count and light count. Surprise makes a new random portal, Vary changes the current one a little, Finish picks new materials, weathering and ground, and the arrow resets the design.
3. The icon tabs hold every setting: Portal (structure, opening shape, size and bores), Details (wing walls, parapet, ring, keystone, cornice), Route (road, railway, mine track or path), Inside (lining, lights and the far end), Hill (ground, slope, trees, grass, boulders and stones), Signs (name, signals and safety signs), Finish (materials, tint and weathering) and Parts (part visibility, detail, Convert to Mesh, Make LODs).
4. Every portal is built in real meters with its origin on the road or track at the middle of the portal face. Each part is its own named object in the portal's collection, so you can hide parts, convert to plain meshes or make three LODs for a game engine.