A spline of yellow/white lights spaced at 15m intervals and 5m above ground when initially placed. They face sideways, so they can be strung along the walls of a rail tunnel.
Getting any spline to follow a tunnel can be tricky. Use the Trainz 'wireframe' view, looking directly downwards and insert as many spline points as you need to follow the curves. Adjust their heights to match the tunnel, otherwise the lights will sit above the ground, not in the tunnel.