Just to see if any further improvement was possible, I left this running for over 18,000 generations. 98% of the improvement had happened by gen 183. The last minor improvement was at gen 7313. There's no sign of a controller evolving that can handle non-flat terrain.

I used to work on legged locomotion. Once you get off the flat, traction control ("anti-slip") starts to dominate the problem. It looks like this simulation isn't going to evolve anti-slip control.