A spline of yellow/white lights spaced at 15m intervals and 5m above ground when initially placed. They face downwards, so they are meant to be strung along the roof of a rail tunnel.
Getting any spline to follow a tunnel can be tricky. I've found the Trainz 'wireframe' view very helpful. Look directly downwards and insert as many spline points as you need to follow the curves. Adjust the heights of all spline points, otherwise the lights will sit above the ground, not in the tunnel.